Rob,
Point 1. If you want people to amend a drop down list I suggest you put the choices into a SharePoint list and use that as your data source. It's much easier to control access and you don't have to worry about version control and installing InfoPath designer onto X number of machines.
If you meant only certain people could see and select from that drop down list, you'll need put the drop down list into a section and control who can view it via rules etc.
Point 2 you can do, yet it's is tricky - and I don' think worth the hassle. There are often too many variables which need to be taken into account. Since you have SP, why not use a discussion board list?