1. Yes your iPhone will remain unliked but of course, the jailbrake is gone along with all apps u installed from cydia/installer (they will be gone too)
2. Because if the whole #1 thing u will have too install it again.
3. Back up with iTunes Anf when u jailbrake with the custom ipsw just restore from that backup
Also you must wait for the new winpwn 2.5 to jailbrake (or use quickpwn) 2.0.2
Winpwn 2.5 will be availabe tommorow.
After u jailbrake u must install your Cydia apps again and u will be on 2.0.2
Hope this helps and reply back for any more questionS
bump.