Hey DBCH, you bring up some good points, I think can answer some of your questions though. Micheal only breifly touches on Roth's motive towards the end of GF II. He simply says "Roth thinks he's going to live forever", Michael implies here that Roth is simply power hungry and wants more power due to his arrogent sense of immortality.

Danny Aiello (Rosatto), saying "Michael Corleone says hello," to Frankie before he strangles him, was as another previous post indicates an improved line that FFC kept because it fit with the plot.

This opens up a whole new can of worms to whether it was a botched hit or made to look botched. If it was botch then Danny Aiello's line was delivered to mislead Pentangeli. If on the other hand it was botched, it can be argued that since Roth was the one that ordered the hit on Michael, he was just taking what Mike said at Roth's home in Miami (Mike's comments about how he knew Pentangeli order the hit) as permission to get rid of Pentangeli who was in the way of his desire for power; i.e. in the way of the Rosatto Brother's who were under Roth's rule.

Also in GF III, I have always just assumed it was Zasa that ordered the hit because he left just before the hit after being snubbed. Due to the holes in the plot, it's really impossible to know if Zasa was just an arrogent and ambitious person or was working for Lucchese.

It definatly IMO was Zasa because what FFC and MP do brilliantly in the GF trilogy is make us beleive person A did this and person B did that for these reasons, but it's really person C who's behind them. Person C is usually a manipulator who aligns the person's desires and goals with their's and manipulates the person in a convincing way to do their will.