Can I shave the bottom of a metal casement window to get it to close fully?
Rather than attack the window itself, I'd re-install it.  Usually there's a gap around the frame filled with spray foam insulation.   Take the trim off the inside to see it.  You'll also see a bunch of screws that hold the window frame to the house frame.  See if you can take them out. Once this is done, you can scrape off the spray foam with a stiff knife.

Stuff the screw holes with toothpicks or matchsticks (take off the heads with nail clippers), and remount the window, making sure it's square.  In most situations, the settling is transferred to the window via the screws and or spray foam -- That is, the house hasn't settled enough to close the gap completely. Remounting the frame will solve the problem and leave your window intact.

If you don't feel comfortable doing it yourself, it shouldn't cost a whole lot to get someone to do it for you.  On the order of $100, Less if you can make it an add on to another job you're having done.