well we can answer very clearly many of your questions
1) Zimbra is a Badge-ware when it comes to open source..means, you cannot change, deform, replace, use the logo for profit and other things.
if you package zimbra with your software without changes in binary and any changes in copy right notices and logo's..you are ok
2) if you change and code and add to the zimbra binary package the you need to release that code to public and also you need to keep the orignal code and logo's and licence intact
3) you can however create your own Zimlet or plug-in software which DO NOT change zimbra code and you can keep that as your commercial offering.
you can read the layman terms of at the following link
Zimbra - Trademark and Branding FAQ
if you have specific questions then feel free to ask
Raj