If I move some applications to a SD card and perform a factory reset, will they be available when I plug the card back again after the reset, even if the device's primary account changed?
To add to @Mattthew 's comment, even if you move the apps to SD card part of them always reside inside internal memory. Also the system needs to know what apps are installed, apps like Market need to know about them. So, no they won't work after factory reset.
For big games like from gameloft the extra data you downloaded from inside the game will work though. I don't know what will happen to this with the new Market policy of allowing upto 4GB files.