Canon SL1/100D has permanent half-press shutter

My 2+ year old 100D has had this issue for a long time so I rarely used it. It thinks the shutter button is half-pressed constantly, making most of the buttons disabled (like menu) because that's the normal behaviour while shooting. I can get the menu up by doing a sequence of mode changes and then I can prove that all buttons work, the touchscreen works, etc.

I tried the isopropyl alcohol cleaning twice and no improvement. Finally I disassembled the camera and tested the shutter switch with a multimeter. Both the half-press and the full-press are detected when measuring each of the pins to ground in a continuity test. This tells me that the switch is not faulty or dirty as I thought.

I tried cleaning the remote shutter port with compressed air in case there was a debris causing a short on the autofocus line. I am next going to try a remote shutter release to see if the port works at all and if it can override the stuck half-press (unlikely). I can still take photos, but it's a pain adjusting settings.

Any other ideas as to why the camera could think the shutter button is half-pressed? Faulty ribbon, contact, trace somewhere along the line? I can buy a top body for not too much and install it, but that might not be it. Or I can get a diagnosis from a local shop for $40 and they deduct that from the repair if I do it there. Or I can send it to Canon where it will be $150+ for the whole repair. But I've already opened it (past warranty) and stripped screws which I've now left out, will they care?