Good luck Capt!
IIRC the pituitary gland is on the bottom of the brain, kind of out of the way compared to the bulk of your grey matter, that might be why its only minor, if the tumor isnt large, isnt invasive into other areas, isnt into the serious grey matter and can be easily reached. Of course no surgery is simple or easy, but some are more so than others. They will probably trim out what they know is the tumour and then test around the edges to check they caught it all - quick bunch of frozen sections to the path lab and if all is well and sow you back up. As for diving post op, i really wouldnt know, but i would assume it shouldnt be a problem once its healed.
You'll get through it!