I have one:

Since a canon knowing about the PPC is related to that canon's ability to break the fourth wall, do all first person narrators know about the PPC? Or is it just the ones who specifically address the audience, like Percy Jackson at the beginning of The Lightning Thief?

Makes-Things does, in fact, make all sorts of things. He is a builder, inventor, tinkerer, hider under desks, and the kind of guy you would want around if you ever needed your starship/time machine/console/hover board/video game fixed.

As to your second question, yes, CADs can be built into any number of things. I seem to recall someone wanting to have one built into an original Gameboy case, though I don't remember who or when. As long as it has some means of reading out the information (a screen or speakers would work) and buttons (which an accordion has lots of) and enough space for the hardware (I wouldn't recommend trying to build it into a silver dollar), then it should be fine.

-Phobos

Makes-Things mostly makes plothole technology, but he does have knowledge of tech from a wide variety of continua. I've seen stories where he works on Star Trek tech and TARDISes, for example. Standard equipment is most of what he deals with personally, but it's hardly all he can do.
There are other DoSAT techs that will be more helpful with technology from individual continua, but Makes-Things is still good for general knowledge and damaged equipment in most cases.

CADs can be modified to go into basically whatever would fit them without breaking. Heck, I've seen an agent who had installed one in her body. She was a synthetic life form from an anime continuum, but the precedent is out there. I don't know who you'd be bribing to install one into an accordion, but as long as there was some way that the CAD could actually still be used(it's hard to point an accordion at something, for example), it could potentially still work.

Of course, the problem of the CAD overheating, exploding, and/or having its parts fused together would come up, so you'd better have a really good reason why your Agent has to have it embedded in a musical instrument, especially one that will be basically unplayable if the CAD melts inside it.