A question about informal fallacies in general

Informal fallacies are arguments which contain one or more premises that aren’t true or supported. Does this mean that they contain an element of truth to them? Does it mean that an argument with a weak but not false premise can add some positive  probability to a certain conclusion? Like if someone says that “if I’m not Spider man then my name would not be Peter. But since my name is Peter I’m Spiderman” then it’s obviously not 100% true but does it  at least add some probability to the claim that they are Spider man? If so then what if someone makes a lot of these kind of weak arguments until the probability becomes almost 100%? Or am I completely misunderstanding something here?

Your initial statement that "Informal fallacies are arguments which contain one or more premises that aren’t true or supported" is the problem.  To say an argument contains a fallacy is to say that there's an error in reasoning from the premises to the conclusion.   It's not saying the premises are false.  An argument is not fallacious just because the premises are false.

The statement "“if I’m not Spider man then my name would not be Peter" is a false statement.   It's not a little false; it's 100% false.  "“if I’m not Spider man then my name would not be Peter" is logically equivalent to "If my name is Peter then I'm Spider man". This is called the contrapositive.  Finding just one counterexample disproves the statement.  If your name is Peter and you're not Spider man then that proves the statement is false.   So a trillion such examples adds nothing.