1. Use FAPman (http://maemo.org/downloads/product/Maemo5/fapman/)

2. Never happened to mine so I don't know how to help on this one.

3. Is it fully brand new? Or was it used before? Because if it's been used before, you might want to reflash the N900 to start fresh (http://wiki.maemo.org/Updating_the_tablet_firmware)

4. Again, this shouldn't be happening (I would suggest Flashing the N900 before doing all this again).

Try using CSSU and see if that makes things better for you and your N900.. (Use the Stable version if you don't want to experience any headaches)..

http://wiki.maemo.org/CSSU

EDIT: Also CSSU's default application manager seems to be slightly faster than non CSSU application manager.. But that's only from my own observations