DC -
I agree with you about Neri's composure during all 3 movies. If you look closely, when Fredo is hugging Mike during Mama's viewing, when Mike looks at Neri, Neri gives a brief view - again intentional or not on Richard Bright's behalf - that he really likes Fredo and is going to hate whacking him, but again an order is an order. Neri also gives a very subtle nod that he will carry out this order because it is an order from his boss.

DC - I also believe that you are right about the timeline about the party issue and Neri's presence. He was probably already sent to see Klingman. If Neri was at the party he could have taken care of Deanna instead of Rocco.
Which also opens the question - who put the dynamite in the car before it blew up if Neri and Rocco are both accounted for. Mike would have wanted somebody very reliable to handle this job as well - again another hole in storyline.

Olivant - I also agree with you about the deleted scenes on the cutting room floor. Maybe Coppola should gather them all together, if he still has them and piece them together. Maybe they would be enough to be a GF IV. This would solve te issue of who would play who in a totally new GF IV.