1) Yes
2) You can have multiple UM gateways, but normally one IMAP server (I will have to check if Cyrus IMAP supports distributed message storage)
3) Yes, because of the Cisco limitations for the IVR on the VoIP call leg – you need to pass all the incoming calls to voicemail through E1/T1. If you can forward the calls from CCM to PortaUM, it can process them.