1. CUWL includes mailbox plus ability to own 10 devices per user, if you only have single device UCL Enhanced for the phones and Essential for analog plus separate voicemail licenses might be cheaper, but you will need to compare
2. Extend and Connect is a feature of Jabber with ability to login onto external numbers, so not sure what you are asking for here, please state your requirements
3. Depending what voip protocol you use to control them, SCCP, MGCP, SIP, H323 they all have ability to configure redundancy, but configuration is different, CUCM controlled for the first 2 vs. dial peers for the last 2
Chris