How could Tom have not known about Frankie Pentangeli's survival?
  • Tom lets his one and only client Michael commit up to five counts of perjury
  • Tom had no idea “Alive -- Pentangeli is alive” until after Michael had committed perjury
  • “Fredo says he doesn't know anything -- and I believe him”
  • Then we find Fredo knew plenty

Besides, Tom sets himself up for [self] punishment! every time. Tom never learns.... among others,
1. penultimate boathouse scene
2. Ola meeting
3. Michael enlisting in the Marines
4. Fish tank scene

Tom doesn't read the situation and wouldn't let go.... even after the dreaded Corleone stare and the decision has obviously been made
Tom can't see that the decision made will remain unchanged and whatever Tom says will make no difference whatsoever

It is clear to everyone but Tom!