Secure Ip domain 0

[vad]

1. Not understand - why you want in central site use 2xAS + PCS? You can save database and use server not like PCS but as AS (when two AS will be broken - never see such cases)
2. About IP architecture - not clear your question. Usually remote site (office) - just subnet. Nothing special requirements for PABX. Private LAN, access via WAN, VPN - no difference. In any case - you have separated IP address pool for remote site. So - create IP domain, manage IP range.