UPDATE:
I stripped my MacBook down. When I got the logic board out there was a smoking gun moment. Tea had leaked in through the kensington lock and headphone jack: there was copious amount of residue covering the connector that hooks the logic board to the power source. It was green and looked like corrosion. I cleaned the entire logic board with 70% isopropyl/rubbing alcohol and Q tips. I reassembled and it's all working fine now.
One piece of experience I hope can make it back in to the logic board replacement guide: it refers to how you "use a spudger to disconnect the fan and temp cables" (or words to that affect). Being a novice with working on small laptop size components it was not clear to me HOW exactly you use the spudger. I assumed you disconnect by wiggling the cable out of the socket with the spudger, but in reality those tiny cables need pushing up from beneath. I broke the bluetooth cable before finally realizing how to do it. Similarly on the rebuild guide, it ought to be noted that the connectors are reconnected by placing them over the female part of the board and just pushing down on them. This is probably obvious to experienced repair guys but to me it wasn't and it ruined what would otherwise have been a flawless repair.
Still I'm not complaining- my laptop is in a lot better shape than before I started!
Thanks iFixit.