It appeared that Five Angels was still under some form of Federal Protection so did it seem odd to anyone that a known high-ranking member of the Corleone Family was allowed access to him? I know that Tom wouldn't be the one to kill him, but I still found something about that strange.

Pentangeli probably requested to see Hagen. Frankie Five Angels knew that he'd have to commit suicide and he just wanted to make sure that the Corleones would live up to their end of the bargain (by taking care of the Pentangeli family).

Couldn't a guy like Frank, who was the boss in New York, have left something for his wife himself?

Well, this is a good one. Frankie would have the right to see his attorney at any time, however I think it would raise eyebrows if Mike's attorney and Frank's attorney were the same person, and the "crime-fraud" exception to the attorney-client privilege would have arisen. Indeed it is curious that the Feds were so non-chalant about this meeting, and that it was not bugged.

This plot lapse aside, I disagree with the premise that Frankie knew he would have to kill himself. He knew the Corlleones probablycould not get to him inside, but he also knew he broke Omerta and there were consequences. It was Tom who led Frankie down the road to suicide by invoking the Roman Empire, and what happened when a plot against the emperor failed. Frsankie then said people's families were saved if the perpetrators killed themselves, then their families and lands woould be preserved, but Frankie was skeptical cause he said that only applied to the higher ups. Tom then implied this would apply to his family, and then they agreed this was a good deal, and Frankie had his way out.

I also thought that was curious. The FBI had Tom in their chart as, in effect, "house counsel"--a functioning, high-level member of the Corleone family, not simply their lawyer. Since they knew Tom had helped destroy Frankie's credibility, and that Frankie was in any event a mortal enemy of Michael's for having broken omerta, they'd have to know that nothing good would have come of that visit. Sure, Frankie might have requested that Tom visit, but in real life they'd veto it.

Directoral license to make a superb dramatic scene.

Exactly. Also it sets up Frankie's death.