This is definitely a sensor problem. The issue going top be tracking down the specifics. But I think you’re on the money here honestly. All of your reasoning is solid. This sounds like it’s either an issue with the “sensor chip” or one of its outputs.
This is a QFN chip with a center ground pad. It’s not a chip that requires a huge amount of skill to replace, and it’s probably worth a go. I would hate it. I will take a BGA over one of these QFN chips with a giant ground any day, especially on a Mac. I always feel like I’m going to roast everything trying to get it off because the ground ppl and on the board just sucks up all the heat you throw at it. Bottom heat helps with these. But a board preheater isn’t something everyone has.
That being said. I would still want to get eyes on that area of the board. It’s designator on the schematic us U5800, and it’s about smack dab in the middle of the back of the board (back being the side that’s facing the keyboard when it’s installed). I’ll try and remember to throw a pic in here when I’m on lunch.