When I last SBF'd... as long as I didn't do a factory wipe, the apps were still there. The only way to be sure is try it. I would still do a backup of everything first.
do you mean sbf or just simply restoring a backup? restoring a backup you had previously made will take you back to that point in time and anything you have now will not be there unless it was there when you made it
If you SBF it will. But the apps you've paid for should be linked to your gmail account. As for the unpaid I think that they might be linked but I am not 100% sure.
Flashing back to 2.3.340 will return your phone to stock conditions. I found it best to backup with titanium backup pro, though some apps don't restore clean and will have to be reinstalled which can be done from the market. Make sure you do the factory reset and cache wipes to allow a clean install. The sd card will not be effected. I would also make sure you manually backup your contacts and save it on the sd card before you begin.