> I am not allowed to replace it on my own as it would invalidate the five year long guarantee provided by the manufacturer. Why is this stuff not considered as well?

They're the ones paying for repairs, so it doesn't seem that unreasonable? That said: If you can prove the car is being maintained according to the manufacturer's specifications they can't require you to go to a brand dealership. That's just not necessarily easy to prove.

Recalls don’t require you to have had maintained the car at the dealership previousy