IF countries do not sign the Geneva Convention they are not held to its standards.
That's not necessarily true. Slobodan Milošević was charged with war crimes under the conventions despite the fact that his actions occurred well before Serbia ratified them.
International law evolves very slowly, and it isn't totally settled where these conventions do apply, but you can't assume that states that don't ratify them aren't bound to honor them. And the US has ratified them, so the objection wouldn't apply anyway.
From a practical perspective, since any charges would have to go the security council, it's very unlikely that there would be UN action against US officials. But theoretically there could be, if the current administration chooses to forego the veto.
Now if you are saying that war crimes can be charged against persons, that is another subject.
Well, of course they can. Supra.
Torture is illegal under U.S. law, so the Senate Intelligence Committee members which were briefed on rendition are chargeable under war crimes: Feinstein, Pelosi, Jay Rockefeller, along with others?
Members of Congress don't generally have a command responsibility. They have oversight, and this report is actually a part of their oversight powers. Additionally, they had duties to keep confidential any controlled information that they were given access to, so it isn't clear what they could have done without breaking other laws.