I don't think Denise Crosby was the sole reason Yar was killed off. They also had no clue what to do with the character. I think the best case scenario would've been to have Yar transfer to a different ship.
Then they could have had Crosby guest star from time to time as Yar rather than Sela. At least that would have spared us another instance of
Hollywood Genetics: people are exact duplicates of their parents (or distant ancestors) at the same age.
Agreed 100% on the stupidity of the Sela character looking exactly like Tasha with pointed ears. That just sucked so bad.
As for the O.P.'s question, it does suspend disbelief a great deal if the character is played by another actor, esp. if the actor only slightly resembles the prevoius actor.
I believe that practice began with some sitcom where the actor died, so they then wrote in the death of the character into the show rather than replace the actor. May have happened in the late 1970s.
Oh, and welcome,
redshirtguy1001.
Red Rum!