I'm not sure I understand that confusion issue. Everything you see in ZWC, you see in ZD, and vice-versa, except for Zimlets and offline mail. However given current complications in backing up offline mail, I'd discourage its use (or turn it off entirely if possible). I wonder about the network burden of ZDC--is it any greater than Outlook? And about the comparison with Outlook with regard to duplication of data, if Outlook isn't caching data locally, isn't that going to increase network activity?
In any case, a big problem with ZDC from your perspective is that it currently only supports one user/machine. (I understand that Zimbra is planning on remedying this.)
Could Thunderbird + Lightning/Sunbird be an option, instead of Outlook or Zimbra Web Client?
Or, perhaps you could just use Thunderbird as a sending agent for "File | Send To", and sync its addressbook with Zindus. See:
Zimbra Thunderbird address book sync ; or directly:
Zindus
If that works, you'd want to minimize the amount of mail that Thunderbird downloads by pruning its IMAP folder subscriptions and turning off as much checking and automatic downloading as possible. Setting it up as a POP account might also be worth a look, in terms of minimizing any download of data.