Tasha Yar would have
gone back in time and sacrificed herself, no doubt about that. But ultimately she didn't
, because history was restored to its original form - where Tasha Yar died at Vagra II.
We are told that Tasha survived and went on to become a mother, surviving at least another 5 years in the past. Where did this Tasha Yar come from? Not our universe certainly, since the events of her personal history never happened
The Tasha Yar that was on Romulus was from the alternate timeline/universe we saw in YE. There is no doubt about this.
Her situation is pretty much the same as Spock's in Star Trek
and Into Darkness