Since disassembly at the hinge has turned out to be a thrash, I'll cut the Gordian knot and simply swap the cushions themselves — i.e., install the new cushions, back and bottom, on the original seat's frame as well as the original seat's cushions on the new seat's frame. By the way, I bought a leather cleaner and conditioner kit by Chemical Guys, a brand with high marks, including for its scent. Seems many conditioners have an objectional odor, but Chemical Guys' product smells like leather. I'm going to clean and condition both seats to a fare-thee-well before I install — or rather, wrestle — them back in the car. I found Chemical Guys' how-to video informative:
https://www.chemicalguys.com/blogs/interior-how-tos/how-to-clean-and-protect-leather-car-seats