Code signining is a control that is intended to restrict the software that can run to only those apps which have been granted the right to run.
Your second question is a good one, but given is context, it is unrelated. If apple signs a python interpreter, they do so at their peril, for obvious reasons.
Yes, and it's still only running an app which was granted the right to run, it's just that this app now has some extra code in it. Since Apple doesn't really inspect the contents of the apps it signs anyway, this grants no extra capabilities.