Having heard the stories, and looked at the course maps, here's a THEORY (i.e. hypothesis based on available information, that would need to be tested) that would very neatly account for everything that's been said (with things that would need to be verified noted in parentheses) without anybody being a liar - minus Dean, who would fall into the "truth-stretcher" category:
Dean stayed at the Hyatt the night before the race (needs to be verified), the "race headquarters," according to the website, which is to the southeast of both the starting line and the first turn. The first turn is just under a mile west of the starting line.
For whatever reason, Dean was running late on the morning of the race. He left the hotel with the intent to run to the start line. On the roads, from the hotel, the first turn is about half a mile away. He ran from the hotel, directly to that turn, and arrived at that point about six minutes after the race had started (would need to be verified by Dean, or somebody who saw him running).
At that point, at his pace, he would have been about six or seven minutes away from the starting line, consistent with his story, and then would have had to run another six or seven minutes back to the point where he was, which would have cost him about fifteen minutes total, on top of the six minutes that he was already late.
So instead of running against race traffic to the starting line (unexpected traffic block!), he decided that it was enough "handicap" that he had run a half-mile to get to where he was, and that he was starting "late," and waited until the last walker had gone through before he jumped into the race. This is where, since he was at just under a mile into the race, at six minutes after the race started, if he was not well-hidden, nearly every runner in the race could have seen him waiting to jump into the race, and possibly, one or more did, which is where the reports that he jumped into the race after it started could be coming from. (This, of course, would need to be verified.)
So he would have passed nearly every runner in the race, starting with the walkers at the first turn, missed the timing mat at the starting line - and nobody would have seen him cross the starting line late - potentially been seen by one or more runners waiting to jump into the race after the start, as those runners were passing the first turn, and still run a believable race, at least in his own mind, given his finishing time and self-imposed "handicap."
Again, not THE explanation, but a theory (that doesn't explain the "extra miles," but I think it might be okay to chalk that up to plain old exaggeration.)
Any info to support/refute is welcome . . .