The concept of your sequel trilogy to BTTF is great and overall it's an enjoyable read. However, there are a couple of things that I had a problem with:
1) Doc, Marty and Jennifer arrives in a 2015 without flying cars and hoverboards. Good job on the bit with Verne posing as a little girl so that Marty gets his hands on the hoverboard during the run-in with Griff, but here is where the problem begins. Marty has the hoverboard, but Griff and his gang don't. Therefore, when Marty gets stuck on the lake, events are very likely to play out differently given the lack of hoverboards from Griff and his gang since they wouldn't be able to crash into the Courthouse mall. And that could eventually lead to a paradox if that incident being prevented changes things for the Doc and Marty from 1985.
2) Old Biff shouldn't have recognized Scott in 1955. Based on y understanding of BTTF's rules, whenever a time-traveler makes a time travel trip into the past that he/she "already" made, the new timeline version of the time-traveler gets "canceled out" and "replaced" by the version from the first time around. For example, the moment Lone Pine Marty departs from 1985 and arrives in 1955, he is instantly reverted to/replaced by Twin Pines Marty (as in "our" Marty). Likewise, the Old Biff in 1955 should only remember the 2015 we see in BTTF 2, where Marty isn't a rockstar and he and Jennifer only have two kids.
