Hey Nell,
1. I want to refer you to our nice, new Migration FAQ to answer your first question :)
2. If you look at the wire frame for our new home page you'll see a section in the middle called "The Latest". This is where recent forum posts will go. I don't yet know exactly how many will show up there, as that will depend on some design features that we haven't confirmed yet.
3. We are planning to create a big, visible forum post on the new site that invites members to list any questions, problems, confusions that they're encountering with the new site. It will also be a place to report tech glitches, as we're sure to encounter some at first. The admin volunteers and I will be checking that thread daily and replying or just fixing issues, as applicable.
Thanks for your good questions!