How exactly does putting a provision in a treaty make it harder to repeal? The same actions that ratify the treaty (for instance, passing an enabling domestic law) can be used to break it.
https://en.wikipedia.org/wiki/Supremacy_(European_Union_law)
As to whether it applies to all treaties or just the EU ones, ... good question. In practice courts haven't really followed it.
Also, just as a data point, I've been working in Brussels for close to 20 years now and seen the evolution. I work as a consultant, so it has been a mixed bag for me. However, for just about everyone else, workers, middle class, business owners, ... it has been a consistently worsening disaster.
Having regularly worked for the EU commission, let me tell you this : any idea you may have that this organization has anything but their own economic interests at heart can be cured by chatting with one of them for 5 minutes.