On the ST4 vs. ST5 issue, the final fifteen minutes of the former could indeed have been very, very long. For all we knew back then (or now, even), our heroes had been sitting on their hands while Starfleet built them a new ship from scratch! At the very least, there would be bureaucracy involved, and that does remain the one constant in the universe... Things like that don't happen overnight.
OTOH, the stardates might be considered relevant, too. ST2 through 5 have theirs back to back in the early 8000s: ST2 is 8130, ST4 is 8390 while ST5 has 8454. This would best work if the four movies indeed took place within a year (even if this year was split between, say, 2285 and 2286). One might even see "writer intent" in those dates. Which raises the question of whether such intent should count for anything in itself.
I cannot imagine any other reason for the 2287 date of ST5 beyond the silly "20 years sharp since first Romulan re-encounter" thing, and I do loathe the "20 years sharp" doctrine of assumptions. On the issue of the Valiant being lost 200 years sharp in the past of "Where No Man", the assumption is even explicitly false, because the ship went missing "over two centuries ago" by Kirk's words. Supposedly, she wasn't reported missing the very day she was launched, either...
However, does the Chronology have consequences in this respect? Has any date in any book been fixed in relation to the 2287 assumption of ST5? Or is it merely a headache for those who want to know the exact date of ST5 and then divide the books into those taking place before and after that date because they take place before and after that movie, respectively? Nudging ST5 back and forth shouldn't be that big a problem, I'd think...
Timo Saloniemi