The fate of the Enterprise-C
will be dealt with in STO's upcoming third anniversary episode, with Denise Crosby providing voiceover for Empress Sela.
In STO, the Romulans captured the Enterprise
and this mission will see the introduction of the Ambassador
-class to the game.
Alt-Tasha is probably dead, otherwise Sela would have had it her internal monologue in any one of her Lit appearances. Unless she doesn't know Tasha was kept alive and the execution faked.
I don't know whether it will tackled in TrekLit someday but in less than two months, the Enterprise-C