though you could get a second quote at macauthority. i take it you're out of apple care? (though i don't know if that would cover this).
check the fixit forums.
one of my friends spilled wine on his, and even though it worked fine, the shop told him it was like a cancer and one day it would definitely stop. evidently it keeps corroding.
i've never done work on laptops, but you could also look into replacing your own logic board.
i've found good guides online for such things.
oh, and when shutting down, you can just take the battery out for now.