For anyone having the TYUIO key issue: I'd been having this issue intermittently for a few weeks - insanely frustrating. Read all the forums I could find and tried many of the suggestions - blow out dust from keys, reset PRAM, SMC, Accessibility Mouse Keys, British Keyboard source, etc all with limited success that I was chalking up to coincidence and placebo. Mashing the keys was the only reliable temporary fix.
Eventually I took off the back casing and noticed the center fan had more dust collected on it than the other fan, suggesting it may not be running as frequently as the other fan which is definitely a problem. Powered on the laptop with the case removed and confirmed that fan wasn't spinning, and furthermore the metal of the vent was getting way hotter than the other fan which was running. I removed the non-operational fan, cleaned out the dust from vents, and reseated it and made sure the connector was secure to the logic board. Powered the computer back on this time with the center fan working properly and surprise, the keys work without issue!
The center fan is seated directly under the TYUIO keys. My hunch is that when that fan isn't working, that area heats up to an abnormally hot temperature and affects the key performance. This would also explain why the key reliability is so intermittent, as their reliability changes as the temperature of the logic board increases and decreases with regular use. Could be coincidence, but seems like a rational explanation and a reasonably easy fix. If you have to replace a broken fan, or just reseat the connector like in my case, way cheaper and easer than replacing an entire keyboard or logic board or upper case assembly.
