Stiv,
Answers to your questions;
1. If the call is coming from FXO to IP, then you cannot add different prefixes with the A800. You can do this with the AX/AS units.
2. There is no way to prioritize calls coming from IP to FXO on the A800. The only way to route them to different FXO ports is to have unique LAMPatterns.
3. Not sure I understand, but if you are looking to only accept IP calls from some IPs then you use the allow and bar entries to setup those IP address(es) that you want to explicitly allow or explicitly bar.
Mike