Ooh, that kinda sucks. I doubt everything in the trans draws more than 1 amp peak, until one of them shorts. I'm surprised GM did that.
In light of that information a relay setup like you describe would be my choice on an auto car.
I think the three slots are to give more light transmission for a cleaner high-speed signal at the phototransistor - as opposed to the low res side which switches very slowly by comparison.
I respect everything you take the time to post, but after spending plenty of time researching my personal opinion is that the phototransistor (receiver side) eventually shorts internally from temperature. All things aside, if you put a bunch of semiconductors on a chunk of aluminum that for all intents and purposes has 215F oil sprayed on it it's whole life, failure is inevitable, and I think it's no accident that phototransistor equipped sensors have been almost completely absent from the automotive industry for nearly two decades.
Unfortunately not. Unless these coils have some internal self-diagnostics that we're not aware of it would require monitoring current to each coil. There aren't enough adc channels to start with, and you would need current sensing shunts for each coil which would increase wire and part count immensely. Maybe if misfire detection works that would help diagnose, but I'm not optimistic it will (work). If GM is doing anything like this currently, keep in mind they have a slightly larger R&D budget than Scott's basement lab.