The 25th anniversary achievement can be completed, I did it.
How did I do? I know that for me the progression bar is stuck all the time, except on the inferno scenario number 2, the one with Namtaru. On this one, the bar did progress normally. So... I stood right before her and clicked "end turn" until month 301 (or so). But always save and return to main menu to check if the bar has advanced.
Yeah, I know, I really wanted this title...
Concerning Michael's achievements, I also tried killing him with 4 minions left, one-shot him aswell, none worked.