All good points. I shouldn't kid myself, I know what I did wrong.
I tested the blade angle by the laser sight, cut a 3in end off of a test piece and flipped it around to see if the resulting corner was square. It appeared to be square, but it clearly wasn't long enough to give a good respresentation. Also, I should have held my square physically up to the blade, in retrospect.
I learned my lesson: if it only has to be done once, might as well spend the time on it. This regarding the miter saw setup, of course.
also the boards were a touch warped, which I figured the plywood would correct by being straight and square, then pushing the groove onto it and gluing it. This did not work, and was lazy. Next time I will straighten the boards, too.