Found a fix for the rear seat squeaking. Works 100% if your noise is coming from the hinge at the bottom of the seat back that connects the 60/40 section. From what I gathered from troubleshooting, part of that hinge has some slack in it, and creating just a little bit of pressure near the base of the seat back on one side of the split, will create pressure on the hinge and get the squeak to go away. It doesn't need to be much pressure at all; I just put a trash piece of a car detailing sponge in between the seat back and the carpet of the trunk (pics below showing what it was, where & how it was inserted).
When I had the seat out, it looks like the specific issue may reside with that split insert (possibly considered to be a bushing) closes in on itself and reduces the gap/diameter of the insert. Tested this by taking a flat tip screwdriver and twisting it in that insert's gap to open it back out more and the squeak went away, but then it came back again a few days later. With the sponge in between the seat, it has been perfect.
When I had the seat out, it looks like the specific issue may reside with that split insert (possibly considered to be a bushing) closes in on itself and reduces the gap/diameter of the insert. Tested this by taking a flat tip screwdriver and twisting it in that insert's gap to open it back out more and the squeak went away, but then it came back again a few days later. With the sponge in between the seat, it has been perfect.