Is it a pro no no to simply cut the sheet in half...one extra seam?
Not just another seam, a long hard to deal with butt joint.
I have 2 ideas,
1. how hard would it be to remove the window PITA yes.
2. 1/4" drywall will bend quite a bit. Maybe try 3/8" but won't bend as much. If code is for 1/2" you may have to double up the 1/4"
Did the crack the boards trick when drywalling my parents beach house's wc rooms that were too small to get a full sheet in and stood up. Taped and mudded all the seams as usual, where the board was cracked put a single coat of finish compound to hide the line that was barely noticeable, sanded and painted. That was in 1997. It has held to this day.