There is nothing special about the Assistant phone other than that it has multiple lines (one being proxy, one the intercom and the other being personal extension and possibly a 4th speed dial to Manager intercom. As long as all the phones to which the assistant logs in, have enough line buttons on it (say all phones are 7960)..the assistant should be able to move around using EM.
One problem with this i can think of is that if you are using auto answer on speakerphone for the intercom lines, when you create extension mobility profile, you may lose the auto answer function (as its a shared line).
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.