I thought I would post an update; this seems to be entirely related to the temperature. If it's a hot day, the locks don't work. Otherwise they do. Weird.

Have you checked all the fuses? I’m not sure if your model/year has a Rear Electronics Module (REM) with an associated fuse panel or if the rear accessories are fed by a fuse panel further forward in the car. But your rear door locking issues sound a lot like the problems I was having when my tailgate wiring got pinched/severed/shorted up inside the LH tailgate hinge. This led to blown fuses and rear doors that wouldn’t lock, along with other failures including the gas door wouldn’t open in the gas station until I popped a new fuse in.

Have you lost the fobs or are they "broken". I have a 2000 V70 (P1 type) my fob was dodgy and found that one of the connections on the PCB switch, lock button, had come adrift. A dab of solder fixed it. Check the 4 connections on each of the 2 switches. Use a fresh battery.

Before taking the doors apart, I would fiddle with the individual door locks in the rear doors and the child locks in the rear doors - just an out of the box thing to try,

Probably may not do anything, but something easy to try.

Doh! - Until we got our used '88 745 GLE, in 30 years or so I never even had any of those features.

Spraying the lock will do no good. Did the passenger front door unlock when you turned the key the second time? How about the rear hatch?

Try pressing the lock/unlock switch on the inside of the door and see what happens.

If nothing happens when you use the switch in the car, it may be a loose wire.
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Keeping it running is better than buying new

You need to remove the door panels to see what has failed.
It may be as simple as some broken clip to support the link from the lock motor
to the door latch.