I had a second try putting the remaining pair of wheels in the oven.
This time a gave it 10degC more (160degC), pre-heated and a full 15 minutes.
When I opened the door, I was a bit worried because white smoke came out.
This time I did wore protective gloves, and again the outside of the tire came of easily. And the inside would not move a micron! Really well bonded.
No idea why this is, nevertheless I have saved both rims and tires.
Check out the amount of sand and debris, which came out! Amazing, is it not?
Downside is that the two closed cell inserts are ruined because they had been in the tire for more then six months with this crap. When you squeezed the tire, you could not compress it. Here you see why.
The other set's inserts are perfectly fine.
Maybe I put in some new ones if I can get my hands on a set soon.
But surely I will close the venting holes in the rim, and puncture four small holes in the outside of the tire.