OK, as my favorite TV "criminal lawyer" likes to say, let's get down to brass tacks. Here are some thoughts on the possibilities I listed earlier, and why they fit (or don't).
Suicide
I don't think there is nearly enough information to be able to rule this in or out. We frankly don't know a lot about Paul. He's married, has two college-aged kids, likes hiking, goes to church, did I miss anything? If he's sick, we don't know. If he's having problems that he's working through, we don't know.
And to be fair, there are reasons why his family and friends wouldn't be broadcasting that during the first few days of the search when he was most likely to be alive - one, his dirty laundry is out in the press, and two, there's a chance that people will say "ok, he went to die surrounded by natural beauty" and searchers won't risk heat stroke looking for him/fewer people talking about him/etc. Sad but there is a bit of truth to that.
Death by stranger
I still don't think this is likely. The police don't seem to have found anything; if they did, the search would have been pulled back much earlier; as it was, people were risking heat stroke being out there. Walter White is not going to be cooking meth at the trailhead to a semi-popular trail just outside town and killing anyone who comes by. The hiker who claims not to have seen Paul didn't seem to see anything else notable, at least not that we know of. (Though I'd like to know if he saw Paul's car.) They didn't take his car or his phone, and as far as we know he didn't carry anything of value with him (expensive watch or camera, etc.) that could have tempted a thief enough to use a deadly level of violence.
Walkaway
I think this is possible (Paul parks his car at the trailhead and meets someone to run away with, or walks close enough to town to call an Uber who will keep quiet with enough $$), but I don't get a motive. There aren't all that many adult walkaways, and usually there is a reason, like severe family problems (Kimberly McLean/Lori Erica Ruff), mental problems (Robert Ivan Nichols/Joseph Newton Chandler), etc. Not something we've heard about. If he wanted to get divorced, he could get divorced, nobody cares. If he was coming out of the closet, nobody cares. If he was afraid that his close friends at church would judge him, they're not going to be in his social circle anymore anyway, regardless of what he does. The kids are college age so there's no child support, and a 51 year old man is going to have a tough time in a foreign country getting a job.
Actually missing
One of the articles about this trail mentioned what a searcher had referenced about the only way him not being on the trail being completely obvious - specifically it's walking on past the oasis at the end of the trail. Everything else seems to be a canyon. No footprints, he didn't leave clothing behind or any evidence that he was here in the hopes that searchers would find something and track him down. I assume they've looked for footprints to see if he wandered off in another direction from the trailhead for some reason. Of course it is possible he's on the trail and was missed, but from the way the local SAR people have described it, it's almost impossible unless he was being deliberate about it, which means it was either suicide or intentionally risky/dangerous behavior, to the point where it may as well be suicide.
Wife killed him
Their kids are college aged, the youngest is 19. I know more than a few couples who got married, had kids, bought a house, became empty nesters, looked at one another for the first time in decades, and got divorced. Or stayed together only because of the kids. And we certainly know of cases where things started to get ugly, especially when it comes to division of assets, and one spouse ends up dead but it's staged to look like something else. Sometimes even for the insurance money.
We don't know right now if Paul was sighted at the hotel near Joshua Tree. Specifically Paul. The stretch of road between the Grand Canyon and Joshua Tree has a lot of nothing, plenty of places for a body to be dumped and nobody would ever see it.
Why the trip? Maybe the couple was having problems and this was a chance to rekindle their romance, or to give it one last chance before calling it quits.
Insurance fraud
I mentioned that cases like this have happened before, where one spouse plays dead and the other collects on the insurance money. Here we have a classic case of this if this was the plan - they won't find a body as there is no body to find, the chances of him actually living if he was on the trail are increasingly slim, and good luck proving he is alive. Paul may have driven to the trailhead and walked out, with the plan that he eventually meets his wife somewhere, perhaps back in Canada after they move away from Guelph, or to another country where a $1M policy will provide them with a comfortable living and that won't extradite for insurance fraud. He'd almost certainly be flagged if he crossed an official border back to Canada, but an experienced hiker may be able to make it in a more isolated part of the Canadian-American border, one that may not be heavily patrolled due to remoteness for instance.