"Even in Yesterdays enterprise where the Captain of the Enterprise C dies and Tasha joins the crew due to the timeline changes the timeline as history recorded it still happens. The Ship is destroyed and the timeline unaltered as far as people know."

Uhm, maybe that's because the Enterprise C traveled to the future and not the past? Spock went to the past*. And alternate universes are nothing new to Star Trek, if you recall that one with Worf shifting through dimensions.

*Just realized this point is invalid as Tasha goes back with them, thus restoring the timeline... gah!