If you are able to remove the screw from up top the handle should come off so that you can replace your cartridge. You made have some difficulty removing it like this and also replacing it properly if you do get it off. The handle will seem loose. If you can get it off maybe some (WD-40) to loosen it will work. Also FYI 1-800-BUY-MOEN and you can order replacement parts from them for FREE!!!
Glad you got it out! The link I sent should take care of your other questions. As for the hardward store, yea there is a chance they can match it up but it won't take to long for the parts from moen for free either.