I'm pretty sure that the side with the groove cut in it is the side that's supposed to come off. Old XT-level and above bottom brackets had the other side stuck on with some blue loctite-type stuff. They were never stuck on all that tightly when they were new. You could usually hang the BB from the jaws of a loose vise (with the flanges sitting on the jaws) and gently tap the center section out with a hammer.
When building nicer bikes in the shop, I'd tap the "fixed" cup off and grease the inside. This not only prevented the situation you ran into, but it also helped prevent squeaking that could sometimes develop in there.
Speaking of which, I wouldn't go too crazy getting the corrosion off. Those cups are supposed to fit snugly. Once you tighten the cups down, you should be ok, but any free play is asking for trouble. I'd just grease it liberally and put it back in as-is.