I have a mid-2014 MacBook Pro with the similar issues described here. Wouldn’t power on at all. Took it to a authorised repair centre. They opened it up, blew some air in, dis/reconnected a few things (unfortunately not sure exactly what), and it started working again. I took it home, worked perfectly. The next day, dead again. Took it back to the store, they couldn’t get it working for several hours, about to give up then it came to life again. They ran diagnostics on it for 24 hours and could find nothing wrong. They said when it wasn’t working, they tried booting via the onboard power terminals and that still didn’t work, so they thing it’s logic board related rater than the top case. Kept it in the store a few more days. I’ve just picked up again, and ‘seems’ to be fine. Will see what happens next… would love to know if anyone found out the cause.