This is going to be long considering I've been storing up my comments while reading about 100 pages! I can't believe all the discussion about how similar this guy looks to MH...even though he's got honking tattoos or a scar on his face! Obviously not him, don't post and move on. It seems like way too much energy is being put into theories that sound like good tv shows or movies instead of really understanding who MH is. I'm a computer expert for several decades, graduated top of class, won only scholarship for upper year comp sci, etc. I led development in North America for one of the biggest software companies in the world. I'm saying this so there's some credibility behind what I'm going to say here. What I can tell you about this guy is:
1) I have looked over all of the transciptions of his notebooks or so I think. Contrary to some who I'm sure have good intentions but, frankly, don't know enough about what they're talking about, the majority of it is a needs analysis and specification for a game, mod or add on for a game, almost surely Screeps. I ran across Screeps after reading his notes and searching games with phrases, words, objects, etc. that his notes used but then I read that others had already come to this same conclusion and tried to get them to help...to no avail so far. I would say he's done it decent to good, intelligent, thought out and proper. There is no rambling, no mess nothing odd, as long as you're used to reading and writing that stuff. Almost surely, he worked in the field and not for tiny businesses. He may have worked as an independent software/IT consultant, at a consulting company or at a company doing their systems. He probably did work for, not tiny clients and probably not enormous clients. He almost surely has a formal education in computing to an extent because of what he's done in the notebook including using proper coding standards such as indentation, spacing, etc. I marked assignments in university for comp sci students and 60-75% of them didn't format and write their code properly as this guy has. I haven't coded very much in the past 15 or so years, you start out as a programmer and I've done my time at that. Still he's coding in C/C++ or a language that is based on it, probably Java. Many of the languages can be ruled out such as asp, php, fortran, apl, cobol, modulus, pascal, ruby, python, etc. There are two main worlds these days: Windows and Linux. I would venture to say he's more in the Linux world using Java, Tomcat, Apache server(s). These are small sets of computer people

He could be involved with open source projects too. These are his notebooks that he wrote. This is not just info about him while on the trail but his background is displayed in those notes! Much more background than anything else we can find. About the only other parts of his notes are a little chunk about nutrition which is oddly placed in the middle of the Screeps specs but maybe that's just when he was thinking about it or how they transcribed it. It looks like he was intelligently trying to figure out how he was going to maintain enough protein in his diet without carrying 20 pounds of meat and keeping it good for weeks. The other part is ideas for a new game he's creating called Nanites Guild or something, haven't got it in front of me right now. In short, his notes are fine, there's nothing odd about them and they say a certain amount about who he really is beyond the hiker.
2) He said he was in IT for years and he most likely was. People saw him in New York first and he told people all along the way that he was from New York and started there. He probably was and did. He told people he was only travelling 10 miles a day and he seems to have been moving at about that speed so he probably was. What he said all along the way for over a year to many people, even people he stayed with for days is all consistent. None of it sounds exaggerated, more the opposite as people have said he was friendly but seemed shy. Nothing seems nutty, manic, depressed, schizo or anything. He wasn't popping out of the bush every few days to find a drug dealer or grab a few bottles of the hard stuff. What I'm getting at is the most likely scenario is he's telling the truth in most to all of what he said. He probably does have a sister. She probably does live where he says. If he was lying and said he was staying with her, why wouldn't he say she lives in a close by town to where they were at instead of 3 hours away? His dad was probably physically abusive and he probably hasn't had contact with his parents for many years. He may have lost both entire families by distancing himself from them. It then wouldn't be a surprise that none of them would be looking for him...except maybe his sister. Not if he's gone off on these tangents before though. She may just think he's off on something and will return in a few years. We have no reason to think he's lied about anything unless and until someone can show otherwise.
3) Speaking of tangents, he's a gamer or he wouldn't have those writings in his notebooks. He's gone into different and contained worlds before under a different identity, with an alias. This is what he seems to be doing with his hiking. This would seem to be a self-contained journey for him. He expressed to hikers he didn't use social media and stay connected as he thought hackers might get into his stuff. He did the gaming online but I'm leaning toward thinking he didn't do much else online. He wrote in notebooks and seems to be reading, in pics of him, so he may have done more of that. He was found with near $4,000 dollars on him. I would propose that he made a conscious decision to assume a certain identity with the alias Ben Bilemy for his journey and carry cash without ID, bank card, credit cards, no other identification, no keys, no emergency contacts, no access to anything else. After all, if you're carrying that stuff then you wouldn't be carrying so much money. It's one or the other. Robbers could only get that money, not any of his possible wealth back home or identity. He didn't come up with Ben Bilemy on the spot at a hostel check in desk. If someone wants to fade away, they'd use Jones, Smith, John, Bill, Not Ben Bilemy! That name means something to him. To wrap up, this was his purposely separate hiking life he quickly adopted near the beginning and stuck with.
4) Almost nobody goes into the woods alone for days carrying thousands of dollars and, if he had near $4,000US at the end, let's guess he had 6,000-10,000 to start. Almost nobody would take all of the last money they had either. In fact, that money probably wasn't that much to him or he would have made some kind of arrangements beforehand to not be carrying all of it all the time. It makes me think this guy made very good money and $10,000 wasn't that much to him. He also seems very well mannered as some have said and seen in some pics. So, his parents may have been either doing pretty well, regimented such as army or both. He didn't grow up in a barn, as they say. He could have played the software developer, project manager, software consultant on the front end with clients very well.
5) There has been discussion about how nobody could have realized he is missing. There are plenty of people who have gone missing without anybody realizing. He probably quit his job as he said, so who from work would be looking?! Estranged from family. If he gamed lots, worked a fair amount, went off on other tangents and hardly hung with friends, who would miss him? He's just off doing something for a few years I guess. Sometime he'll pop back in. If he led the life I did in the software company, it was more than a job, a way of life much of the time with flying around, staying in hotels, going out with whoever you met during the day, etc. Busy. Little to no time to hang with friends.
I hope I've done something I can do best in this case and that's make a proper and realistic high potential profile that goes far beyond what I've seen or read so far or what the police could possibly even have. With it, I am also saying, how about we concentrate energy on who he most likely is and what he was most likely doing instead of all these far fetched theories, probably incorrect assumptions, accusations of lying, suggestions of criminal activities or criminal family and all. This guy, MH, was probably Fully Harmless and who he said. Just because we can't find his sister doesn't mean he's lying about having one or there's some sinister reason why she hasn't been looking for him.
Where to look from here....
There may have been a period where he was testing the hiking waters and used his real name. Maybe that first few weeks or so in New York, April or May or even before. That log in these posts has entries for Apr and May but just says that he said he did things in those months, it doesn't say people witnessed him. Nobody seems to have said anything about him back then so he could have been going by his real name while visiting hostels, going into stores and hanging with others. He seems to have bought all the gear he used for most of the journey later too so he'd be wearing whatever and looking however. This could explain why we aren't finding people talking about seeing him in those early days. Some people had to have seen him. Who gave him the Denim name?!
Again, realizing the most likely scenario is he was being honest then he probably was trying to get to the Keys and not kill himself the most difficult way imaginable! As far as the end, if he lost that much weight within 2-3 months, he must have been sick somehow, whether he caught something such as from the water or flu from someone else. He got too weak to travel and was probably just trying to sustain, going to get water as close as possible and hiding away again, hoping to feel better but wasting away. I believe there aren't nearly as many hikers in the summer on that FT so then what is the estimate of how many would have walked by him? How many out of how many would stop and see if someone was there instead of just continuing on their way?
Finding the first place he used Ben Bilemy would mean anywhere before, he was using a different name, probably his own while wearing jeans

These people, hikers will be more difficult to find because they won't recognize Denim, Mostly Harmless or Ben Bilemy.
What if he had just found out one or both of his parents had died and maybe it sent him soul searching, quitting his job and hiking?