Very interesting points in both your posts there, particularly the observation about TR. She no longer appears to be a member of the group - although this could be to do with privacy settings too? I have taken a look and it looks like her mother, JM, is still in the group and actively posting. What I found interesting is, looking at her friends list based on your comment I noticed that Frankie's mother shows up as a 'recently added' friend. A search for TR on the group also reveals a screenshot of a long tribute post to Frankie from a couple of weeks ago, presumably screenshotted from her personal account. I think its unlikely there's anything to this, but I did find the wording of her tribute interesting.
I think if Frankie had bumped into someone and decided to go to their house in Caerhun that it is very unlikely that he would have dumped the bike to do so - Caerhun is only a 25 min walk from where he was last seen, so I find it hard to believe that he wouldn't have just brought it with him.
Similarly, considering the possibility of suicide - why hide the bike? Wouldn't you be more likely to just dump it where you left the road? And why go to that much effort to hide yourself? Is that something which is done regularly in suicides (genuine question - it seems illogical to me but I don't have a lot of knowledge or experience of such situations).
In terms of wild camping - I guess it is possible that Frankie took himself off into the woods intending to camp out overnight and met with some kind of accident/bad drugs but especially as someone who was known to like to wild camp/roam I think it's unlikely that he would have done that without stopping somewhere to pick up some food water at least first. Let alone a coat, or sleeping bag.
I agree about the regularity of the cars passing on that road - a random hit and run + moving the body seems unlikely given the chance that a car could be behind you any minute however it is possible that a driver may not have known that and that the road could have looked much quieter to someone who didn't know the area? There is also the road which branches off to the left from Pont Felin through the forest - this is not used nearly as often and someone could have had plenty of time there without being seen at all. Just to add to your point about someone not moving his body to that area unless it was private land - almost all the land there is private land. There are only a few areas which are dissected by public footpaths (and most of those are seldom used and some are impassable).
I really hope that the police have enough to get them to the answers about what happened, I think that will partially depend on how they ended up finding him after this length of time.