google for 3.1.2 jailbroken software. there is software already jb, but u can also find 3.1.2 and put it on ur phone. when u go to restore hold shift when clicking it
That depends. If it's a 3G, then it should be possible with the "shift-guide". If it's a 3GS, then it may be possible with the SHSH-file if you have remembered to store it on Cydia's server.
But you can not downgrade the baseband.