Multisignature: what happens if the number of signatures exceeds the "threshold" number of public keys?
Then the interpreter checks to see that the next value on the stack is
  0, which it is not:


I am not sure, but pushing OP_FALSE is not a requirement, but some kind a mumbo-jumbo to fix the bug in the very first (and still current) implementation of OP_CHECKMULTISIG :)

In fact, you can push 3 or even more signatures redeeming 2-of-3 multisignature output. Not all miners will accept this transaction, because it is non-standard. But it will be valid.