According to Apple, up to three computers (at one time) can be authorized to play purchased AAC's.

5.

I'm seeing some improvement, but not much because most servers out there are bandwidth throttled for a single connection anyway - they aren't serving at 3Mbps per connection, so you won't get anything faster from a single download point.

Either you run in very different circles than I do or else your DSL is simply not providing the 3mbit. I have 5mbit from roadrunner and I routinely download files via http and ftp at 500k/sec+. It's actually rather unusual to see my connection not being saturated on any decent site. And my own servers at a colo facility are all 100mbit, which can obviously feed a whole lot of 3mbit connections...