Hi Turnbull. You're right, that scene where Michael kills Fabrizio WAS filmed but never used. After they filmed it, the studio saw the daily rush of the scene & deemed it too bloody for the times (remember, this was the early '70's), so FFC left it out. If you'll notice, the alternate scene where Fabrizio gets into his car & it explodes is even shot in "long shot" to pacify the exects at Paramount.

I've also heard about the Questadt thing with Michael. FFC was constantly changing the script every day. On Part 1, there were days when the actors would literally get the script the morning of the day's shoot. Which drove everybody nuts. Brando had his lines hidden taped all over the place (on his desk,on other actors,etc)
Even the last scene in Part 2 (the flashback scene) where all the kids are gathered for Vito's birthday. & Michael says to Sonny, "That's pop talkin'", it was. Those lines were written for Marlon Brando. Brando was supposed to be in that scene, but he had a grievence with Paramount after Part 1 so he said he wouldn't be in Part 2. FFC held out hope that he would show up at the last minute, but he never did. So, they gave Brando's lines to James Caan.