yegads...Impressed with your perseverance..have a feeling this might have stopped being fun. The screw, because of access or corrosion?
If access, a small ratchet with bit, or a right angle screwdriver, either make or pick one up. The small angle wrenches with phillips come in handy. If it starts to turn stop, turn it back, then try again working it back and forth increasing the amount of travel, to ensure it doesn't seize. Corrosion a bit of direct heat-penetrating oil from the back side. As it looks like a round head, doubtful you could get pliers on it.
If you had clear space obvious grind or drilling out. It doesnt show but how is the whole burner assembly attached? Just seems if box out of the way you must be close to removing burner assy?