I agree completely. The Clemenza-Pentangeli switch is a weak plot point in Part 2. Frankie's continued mentioning of the "old days" rings hollow when he didn't appear in the original film. It would have been far better had Pentangeli's backstory been that he had been in prison during the 1940s 5 Families War (which would explain his absence from the original film). Upon his return in the Tahoe scene (in Part 2, when the audience first meets him), he could have been celebrated as a "standup guy," who kept his mouth shut while doing his time, rewarded for his loyalty with gifts of the recently deceased Clemenza's house and his regime. After the strangulation attempt on his life, that he wrongly assumes, was ordered by Michael, then he could complain, "I did ten years to protect the Corleones the first time. I'm not doing it again." Then his testimony at the Senate hearings would have more resonance and be more rooted in his backstory.