Can I mount a window A/C into the wall, exhausting to the attic?
Won't work real well. Could lose more then half of the A/Cs capacity.
Need to make a safety pan that would catch any water that may run out of the unit. So it doesn't damage the attic(2X4s, drywall, insulation, etc).
The attic may be 100° normally. But with the A/C running, it will get 20 plus degrees hotter. Making it harder for the A/C to cool your room.
Although more expensive. A mini split heat pump would work better. I'm guessing your room is also cooler in the winter.