I changed them once one year ago at 30000 miles. the inner left one was
worn out uneven. I made sure the guide pins moved freely (they did) and put
new ones.

Now AGAIN, at 42000 miles. Same pattern. The inner left one is gone.
Again, the guide pins move without problem. But I noticed the following; I
had to pry out the pads out with a screwdriver and a lot of difficulty. In
fact, it seems to me they are much too snug fit to move correctly. They are
hard to push them back in position. For now I have put the same pads back
temporarly, but without the brass wear shims on which the pads glides and
they seem to fit correctly, not too loose.

There must be a reason why the car eats rear pads like this; pins are
apparently not the reason. Any help or similar experience would be
appreciated.

 

what pads are you using front and rear?

 

there could be several reasons for this:to better help you answer a few questions in addition to the one above

1) have the brakes been bled recently and the fluid changed?
2) have you adjusted the parking brake at all recently or driven off with it still on?
3) does the car stop with the front brakes mainly or use the rear brakes more?
4) how aggressive are you on the brakes?
5) is there any signs of leakage on the caliper or in the line?

 

> 1) have the brakes been bled recently and the fluid changed?

I have discovered that the pads were worn seriously before the fluid changed at 48000km.

> 2) have you adjusted the parking brake at all recently or driven off with it still on?

Never drove with it on. When it is not on, the lever feels a bit loose. I don't think it stay on. Any way, if it would I guess it doesn't explain why the pads are not worn out even. Only the left inner one is a problem.

> 3) does the car stop with the front brakes mainly or use the rear brakes more?

It brakes very well, no pulsation.

> 4) how aggressive are you on the brakes?

I never warped my front brakes; they are at about 30%, normally worn out.

> 5) is there any signs of leakage on the caliper or in the line?

Everything is dry.

 

honestly, it might just be a good idea to take apart and lube you caliper pins and piston/sleeve. you might discover your problem is with the seals around the piston or a broken c-clip