Hey.

1. YES. It's almost perfect. My previously used full keyboard devices: Nokia 5510, HTC Universal, BenQ P50, Nokia E90, Nokia E71. N900 is the best for me for years now. It has a very nice grip, and I dont miss any more lines to be honest.
2. As it answered before me, you just need to press the char/shift key once, or press it double, if you want it to lock.
3. Well it's Linux, everything is possible.
4. If you aren't connected, main battery drain is the screentime. If you're using it ~half brightness, it can last pretty long with a good battery.
5. Repos will never die, I guess. Some nice dudes are always mirroring them, but yeah, you can install from .deb too.