You can, but should you? If you sign this you should be prepared to explain when you will use them. Assume the opposition will check signatures and look for anyone who isn’t from there to try to use that to discredit this. You should be prepared to for this to show that you are a supporter for good reason.

In the worst case the opposition might even find someone from elsewhere to sign this just to call this out. Often - but not always - the opposition is more ethical than this. (if you catch them in the act of this that is worse for them)

Yep. There’s effectively no location requirement.