Well, it's got nothing to do with canon, but I did include a sequence in Department of Temporal Investigations: Forgotten History that showed events before and during "Yesteryear" and offered answers to some of the questions raised in that episode.
That's a hard question to answer. TAS stories are just TOS stories with an unlimited set and FX budget, better aliens, no Chekov but more Sulu and Uhura, somewhat less sexism, and a lot less death and smooching. They could go pretty much anywhere TOS could go and beyond, except for love stories and violence, and I actually prefer having less violence (particularly since so many of TOS's fistfights and redshirt deaths were gratuitous and added nothing to the stories).
As I've said before, what I really wish we'd had was a second Filmation animated Trek in 1980, as a continuation of TMP. Filmation's animation and music had gotten richer by that point, so it would've been more impressively produced, and it could've had writers like Michael Reaves and Marc Scott Zicree (who were with Filmation at the time) and maybe Diane Duane and Paul Dini (who came along a couple of years later). And it would've given us the TMP-era adventures that we didn't get once TWOK did its semi-reboot. And it's something Filmation could've done, since they'd done revivals of other series like Batman and Fat Albert. So it was a huge missed opportunity.
