1.- IE Plugin is now not a separate download. When you install Server make sure you click on the advanced installation and select the QlikView Clients
2.- Publisher is an additional license to Server. If you license already have it, you still have it. As in version 9 there is no need of additional installation.
3.- It's highly recommended that the version of all servers and clients is the same, so yes, they will need to install it again.
4.- Answered in 1
5.- Not that I'm aware of. Anyway, both Connector and Server release notes are worth reading.
6.- There is a step by step guide on how to upgrade in the Release Notes. Make sure you follow them all.
Hope that helps.