I was lucky enough to have a more quilt-experienced friend to advise me to move my furniture and tape the backing to the floor super-taut. I used high-tack blue masking tape to stretch it a bit at a time, then layered and smoothed the batting and the top into place.
I pinned starting near the center, then towards the margins in quadrants, a pin in each seam and corner, and more along the edge.
Anyway, that's it for now. Anyone struggling with how, exactly, a gigantic, room-sized quilt gets pinned together in the middle of the night before a preschooler can ix-nay the ork-way, now has a bit of a heads-up, complete with nighttime photos.
More soon! Thanks for reading.