Well, having spent a day or two now digging into the Mer documentation (such as it is), I've gotta say yeah, Mer is very nearly exactly that: everything you need to set up your own mobile device, except for the GUI. Really, I'm kinda surprised there haven't been more Jolla-like entities popping up, as Mer really does hand you a significant amount of power seemingly without constraining you to a particular type of UI or a particular corporate entity...