The Mon Cal have a rep of being good starship engineers. I have no problem with a small, maverick design/build team putting the B-Wing prototype together in a garage. I need to dig around, but there was a story about a decade ago about a guy who wanted a Lamborghini Countach but 1) couldn't afford one, 2) he wouldn't fit in one, and 3) they were kinda crap. So he, being of an engineering inclination, got some drawings, turned them into proper plans, scaled them to his height, and built the whole thing in his basement -- frame, suspension, aluminum body panels, the works. And ended up with a better-put-together car than the original production item. So it's not remotely outlandish to me. It's not like Quarrie was inventing hyperdrive or laser weaponry to go along with it. I imagine there was a lot off-the-shelf in there. Which, incidentally, is why the Air Force went with the F-22 over the F-23 -- more already-available parts.
And I don't get the impression there's a "plethora" of Rebel cells out there. Maybe a dozen or so small groups lie the Ghost crew, and a couple larger networks.
--Jonah