I'm afraid I have to concede defeat. 

After nearly 4 hours of WD40, there is no change.

I tried to get the metal frame out of the plastic bit, but now the teeth are not aligned properly, and I can't rebuild it altogether. Damn it!

I won't be able to work on it this week-end or even the week-end after. 

I think I will just abandon them and try to get new ones... What do you think?