Bonjour Catherine,
Possibly, the shaft of the ball could be bent, but that is rather rare.
I’m pretty sure that should work and I’d try exactly that first.
With that age, it is just normal that threads start to rust and seize.
A good knock with a hammer on the key directly (in bolt axis direction) might help, also applying (moderate, ~200°C) heat.
Try to find a matching piece of tube to elongate your hex key!
I personally do not trust WD40 and prefer thin gun oils.
***Worst case scenario here:
If the bolts brake, you have to replace part 216 - not a big deal 
I guess you mean the blue dots on the original bolts?
That is a special sort of dry thread locking compound.
It can be replaced (without any problem) with liquid thread locking compound, as like:
Btw - some of this stuff is present at your bolts right now - and it looses its locking force at ~ 200°C.