1. It has up to flash 9, full web
2. Yes to all
3. 2.1 is released; portrait mode only in some parts, not all.
4. Not sure, but I believe all audio is directed to transmitter
5. Yes to Bluetooth, not sure about usb
6. Default video player has trouble with some codecs even with codec pack. mplayer should take care of the rest
7. I think it's easily adaptable. Tricky doing shortcuts though.
8. Google Maps thru web. Ovi Maps on n900 is a shittier version of the Symbian one. But there are several other navigation apps available too.
9. Battery can last up to 2 days depending on usage and configuration. Phone functions pretty good though it might be some what laggy switching to phone mode.