CXN follows the CUCM software compability matrix except that CXN 2.x corrosponds to CUCM 6.x. The only way you can get directly to 9.1 is if you're running 2.1(4) or 2.1(5); otherwise, it's a two-hop upgrade.
Yes you could use COBRAS Briefcase Mode to import the mailboxes from one into the other. You would need to upgrade both servers to at least 7.0 for COBRAS to work though.
By far the easiest course of action here would be to build a new CXN 9.1 instance and then use COBRAS to migrate mailboxes and call handlers from both systems forward. Again, the source servers must be at 7.0 or better for the APIs to exist.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.