I'm also surprised by the meek reaction in the UK. By now it is clear that during this transcon he is more con than trans. There is also the two mutually exclusive background stories that should raise a flag to anyone keeping a slightly open mind.
I think what we need is a list with claims (with source of the claim) and for each of them the evidence supporting it. Some will turn out to be true others certainly not. Like his London Marathon looking legit based on the splits and a 5-7 minute interview in the final 5K seems plausible.
But at this point I doubt everything until backed up by evidence. The most interesting parts to investigate are the "without sleep WR" and the RAUSA. How much evidence do we have for those? At the moment we are having way to many testimonies of bursts, but very few people that really ran sustained periods of time with him at high pace.
For the UK people some research in his background story (including PRs) and clarifying the inconsistencies and creating a reliable timeline for the without sleep stunt would be the focus. Both for the skeptics and for the believers. If the believers manage to provide decent proof for those claims that would help his credibility a lot. A lot more than calling us haters and pulling the children and nice guy cards. According to me, Kevin tried to accomplish something in that direction. It is a pity he got such a rough treatment as if he was the official propagandist of RY.
Additionally, evidence for each of the runs on his streak would be interesting. Especially the ones where he claims more than just having covered 42K in 3.5+ hours.
As for James and Mark believing him, I'm not sure they realize the kind of claims he is making. I know Mark only as organizer having run one of his races so will not comment on his perspective. I've met James more often and while he is nice guy (see a pattern here) with some long performances to his credit, he is not exactly a fast gun. For a 4h marathoner, both 2h40 and 2h20 look really fast, while they are both in completely different worlds. I'm not sure that he really feels where running fast becomes impossibly fast.