That's a good scenario, Dr. who fixed Lucy...
Except that since the John Cazale had been dead many years by the time GFIII was made, there could have been no such scene.
If Fredo HAD simply been banished in Part II, then I'm sure they would have inserted some bit of dialogue explaining how he and Michael somehow reconciled prior to Fredo's untimely demise at the hands of.....

.
While of course, stepping in to take a bullet meant for Michael.
Apple