If I create a transaction (on armory version >= 0.91) to spend coins to a P2SH address, will an offline v0.90 armory installation be able to sign it?
Good question, I haven't tried it! My guess is: yes, but the interface may not be all that useful, probably "[Non-standard address]" instead of the address string you're expecting. That's something I'll have to investigate... (feel free to try it and tell me what happens)