Is it normal for an unaltered male feral cat to enter an unfamiliar home just to attack a domestic male?

Yes, it's normal. They've probably been fighting over territory ever since they were aware of the other one. Mr. Grumpy is not afraid of you - you feed him. As you do TNR you know that this means you need to make sure your cat is up to date on ALL shots, and check him periodically for feline leukemia and FIV now. This rivalry is bound to continue. Better make sure the plastic is really secure now. Is there some product people use to keep the window from being opened by burglars? If so, you might want to invest in that right now. Or at the very least, hammer some nails into the window to secure the plastic from the inside.