What's more, this was a decade after the series ended. He may have not been able to deliver TOS-styled Spock, anymore. TAS certainly would seem to suggest that ...
Huh? I think Nimoy's very cool, deadpan performance in TAS is closer to his third-season interpretation of Spock than his later movie performances are. And the only TOS cast members who gave good performances in TAS were those with prior voice acting experience, like Doohan and Takei (and Nichols through her singing experience). The production and the recording process were so rushed that the actors didn't really get a lot of direction, so those without voice experience didn't have any guidance in how to do it effectively.
Of course, the whole point of TMP was that Spock underwent a fundamental change in his worldview. He initially tried to be more cold and emotionless than ever, but then realized that emotion had value and should be accepted. And in the final act of the movie and all his subsequent performances as Spock, Nimoy has played the character more expressively, as the more mature and serene version of Spock who manages his emotions but no longer fears or represses them. So it's not that he "forgot" how to play Spock. He evolved
Although it does seem to me that in his two most recent, post-retirement screen appearances in the Abrams films, he hasn't really been playing Spock at all; he's just come in and been Leonard Nimoy.
Hadn't considered about Nimoy's TAS work, but I just checked a few eps, and he really does sound like 3rd season too-cool-for-school-Spock ... and it works just fine for TAS too.
I'd agree about Nimoy's choices to play different from TWOK onward, and furthermore I'd say I agree with nearly all of those choices, up till TUC. The part where he slaps the phaser out of Valeris' hand was just a pissed off guy, not a half-Vulcan (and that may have been the point, but in the past he'd've found a way to deliver the one part without obliterating the other part.)
I think the whole Sybok thing in TFF was something where Nimoy just had to go with along with it without actually believing it, and as a result that doesn't quite always work either (the cut scene where he is talking like a little kid is a pretty strong example of pushing WAY too hard, like Barbra Streissand auditioning for Princess Leia on SNL), but I always give TFF plenty of leeway because I love it so dearly.
I thought he was godawful in UNIFICATION, but part of that is having two recessive performers on screen, and Stewart/Nimoy makes me think Hunter/Nimoy would never have worked in the dynamic compelling way Shatner/Nimoy does (then again, almost nobody has their chemistry, and I can't think of any TRIO that works as well as those 2 and Heavy D.)