Obviously, the OP has the problem well in-hand. Since OP was bringing the rMB back for the mushy key anyhow, might as well get them to look at the trackpad.
Now, for everyone else having this issue, a few things:
1. The new trackpad is, perhaps, the most sensitive trackpad ever made
2. The new trackpad is, perhaps, closer to the space bar than any laptop made in the history of mankind
3. The space bar is the most commonly used key in, perhaps, every language known to man
4. The "inadvertent touch" function is now buried in OS X, supposedly on by default since gestures were incorporated
5. We now have new hardware and a new OS. Sometimes it takes a while for Apple to get around to adjustments
6. This is a hard problem to diagnose, as it's dependent upon the habits of the typists. Me, I'm a sloppy thumb-er who has dealt with this issue on many platforms. And yes, I'm now dealing with it on my rMB. And a very slight palm swipe is probably indistinguishable form a digit. Yes, they do a good job at palms resting or squatting on the pad, but a slight swipe may be another things entirely?
7. If it's driving you crazy and you type with swiping thumbs\palms as I do, disabling tap touch is a workaround. yeah, it sucks, but turning it off will avoid the problem, until Apple tweaks the inadvertent touch capability. I know, it's weird because the symptom seems more like a swipe than a touch, but it works.
8. One other wildcards that can exacerbate the situation can be a bluetooth mouse/trackpad nearby with a low battery. Apple also suggest running First Aid on the volume, but that may be grasping at straws? They also point to 3rd party power adapters and other weird gymnastics, but that's never been the case with me.
Been through this before in both Mac and Windows worlds. Software updates usually wind up helping. But this is also physics: sensitive trackpad + pad closer to space bar than anything ever made + space bar most used key + new hardware and OS = unpredictability for some typists.
This is highly dependent on the habits of the typist. And I said thumbs, but I also think that very slight swipes of the palm may be at play here, at least with how I type and what I've seen. This is a tiny setup, and the proximity of the trackpad to the KB makes it tough. Again, I've dealt with this issue on many systems, probably because I suck so badly at typing.
I've also seen software updates fix this problem in the past. I once had a tiny ThinkPad that exhibited this issue so badly that I entirely disable the touchpad, sticking to that goofy pointer-joystick. Then, one day a driver patch made the problem magically disappear... or at least it made the trackpad somewhat useable.
Again, OP, not saying that this is you, because your system is special. But for everyone else, hope some of the above may help?
