You can have private categories that allow only members of a certain group(s) to read, read/reply, or read/reply/create-topic.
You can add users to a group via a CSV or as part of an import process.
Depending on what you mean by “support”.
You’d have to tell them. One way would be to put that information in the category description.
Search here for “anonymize”
Well, it’s not that friendly, as you have to create an account to see it. 
You can create a topic that tells people where to post.
You probably want to read It's Time We Talked About Tags
As Rafael said, it’s possible, but it’ll be painful. You’ll need to budget 5-10 hours/month for doing upgrades and reading every commit to see what libraries have been updated.
Also, note that there is an importer for your mailing lists.