Was that supposed to be Admiral Hackett? I figured it was just some random person.
To be fair, Citadel never identifies him. The original Asari councilor in the holo-video however is clearly identified since we even have visual confirmation and she quotes verbatin ME1, but I think it was supposed to be Hackett indeed. You can see that the actor was trying to imitate him. And the very first scene of the trilogy has Udina, Anderson and Hackett considering proposing Shepard as a candidate for the human spectre program.
As for Mordin, he indeed seemed a little more introspective, but I never would have guessed that it was thanks to a change in the cast. I though it was just his brain dealing with the big paradigm shift that the reaper invasion represented, finally being able to admit that he felt guilty and made a terrible mistake. I though it was a set-up by the writers from the start, to have him in this state in order to make his big explosion admitting the truth at the shroud a much more powerful scene. Not to mention that he himself admits that he´s been turning old. Had I never bothered to check the credits and learn more from the people involved and I probably never would have guessed the replacement.
By the way, Thane also delivers his lines in a different way in ME3. It´s obvious to me that the writers and voice directors instructed the actor to take into account that the character´s health was deteriorating and his respiratory function is by then much weaker than what it was back in ME2. And yet, it´s the same actor. Since both Thane and Mordin are doomed to die soon (one way or the other in the case of Mordin, since he is old by his race´s standards) I simply though their actors received similar instructions on how to portrait more tired, restrained and contemplative versions of their ME2´s characters. A different actor? Never crossed my mind.