Is there anything else that might invalidate a backup? I suppose creating a new receiving address and receiving coins there is one.
I do not think so - except for encrypting a previously unencrypted wallet. In this case, you even invalidate all new addresses immediately. (Since they keypool is flushed.) But maybe I forgot something here, so please do not depend on that without further checking.
I'm not sure I understand. Don't all wallets start out unencrypted?
Yes. The question is whether you actually use (and backup) it in the unencrypted form first, or whether you encrypt it immediately and only then make a backup and start to use it. You should definitely do the latter.
Is there an easy way to test my backup? Or just delete the install, reinstall, and import the backup?