These are rhetorical questions. You begin a sentence with "Who is to say?" or "What is to say?"
in order to cast doubt on the statement that follows. No answer is expected
The meaning of "Who is to say?" is "Who could possiby say?"
Who could possibly say that we are to blame? No one could.
What is to say? = What could possibly say? Here "say" means "show or indicate" because it applies to a thing not a human being.
What could possibly show that we are to blame? Nothing could.
The infinitive "to say" modifies the subject "who" or "what".
You could rewrite them like this:
Who, to say that we are to blame, is there?
What, to say that we are to blame, is there?