I use this question to drop in some experiences we have made with migrating about 50 users to kopano from zarafa.
For KOE, you need to type in the desired user name, then KOE should find it. Not sure why the drop mechanism does not work. To share something below the mailbox of a user, the user to which stuff is shared needs folder list permissions on the root of the mailbox plus at least read permissions for the item itself (e.g. a calendar).
As for the huge shared folder problem, we have just setup such folders as IMAP folders in the same profile. You need to adjust the send/receive group for that account though. otherwise outlooks spends all its time on checking all subfolders of the imap account for new messages and this delays syncing the ActiveSync Mailbox. Seems to work fine then.
Also, after migrating a company with 50 users I recommend to keep the mailboxes small for ActiveSync, “small” being something less than 10GB in any case. Especially subfolders with many small messages in it (like automated messages coming in every couple of minutes) can slow down the syncing so that the users complain about not receiving mail in time etc.
Resist the idea to first sync only one month to outlook and later “move the slider” to “all messages”. We ended up with some mailboxes synced at the beginning and at the end, but not in the middle…
Last point: consider to deactivate any antivirus plugins for outlook, they caused us a lot of problems, we have a virus scanner on the mail server so we can afford it.