Beneficiary until proven otherwise. That sounds precise. If you are in poor belief that you really are not the beneficiary, I would take care spending the money!

That is how it always has been.

The tax agency can come back 5 years later and ammend You tax filling.

The same here. You announce who the beneficiary is (the ex girlfriend), someone challenges that, but that does not change the beneficiary until the decision has been made.

I don't really see other ways it could work?

> You're not the thing until proven otherwise, the thing has to be proven in the first place now.

You misunderstand. You are the thing until proven otherwise.