I'll take a stab, but my answers come with no guarantees

But first, the server you mention is probably a little 'light' for zimbra. Sure you have only 7 people, but loading up the java stuff (plus mysql and amavisd) can take a bit of RAM, I'd increase that, and you dont' mention the disk subsystem...
1) Not sure about this being "all resolved", but I have a user who is using this combo (Tbird/Lightning/Zindus) and I've had no complaints so far (he says all is fine, except for reserving Resources/Conf rooms, as there is no "Location" or Resource" in scheduling meetings). The alarms do work as I recall, but I believe Tasks are not mirrored/synced to Zimbra
2) Believe it does, yes (see previous)
3) already answered
4) already answered... believe you can do it, but takes some work/manipulation, depends on your skill/knowledge
5) Hmm... zimbra uses/tests/wants the versions that it has, plus they've got this stuff using LDAP. You can disable portions of it through the Admin interface... e.g. you could turn off spamassassin/amavisd on Zimbra, but then you'd still have some work to do. _PERHAPS_ you could dual-IP your server, have your postfix/amavisd/sa run, and then forward to 2nd IP address on server where Zimbra is listening.... I think. But then again, maybe not (as Zimbra might be listening on localhost as well). Hopefully someone will chime in definitively here. But think of it this way... they (Zimbra) do all these nice things by including them. Yes, you lose a little control, but it removes a couple of headaches (IMO).