Remove active connections from backup system after return to normal

1) How can I have a 'main pool' and a 'standby pool' for a Virtual Server in the LTM?

You can use the pool command in an iRule to select the remote CAS pool:

when CLIENT_ACCEPTED {
     Check if the VS default pool has no active members
    if {[active_members [LB::server pool]] == 0}{
        pool remote_cas_pool
    }
}

2) Is this behaviour (of having to write a irule for switchover when the Higher Priority Member comes back online) specific to Microsoft Exchange or is this how it is with the LTM and other applications as well?

It's not specific to Microsft Exchange. This scenario could come with any application where clients leave connections open for long periods of time.

Aaron