I remember to have seen a story in France of someone that had bought a house, but someone else did break in and rented it to a family. The people in the house had a rental contract, so they had the immediate right to stay, even it the contract was invalid. Evicting them was a nightmare.

Here is a similar story: https://lemans.maville.com/actu/actudet_--nous-sommes-abando...

Absolutely. This is for secondary houses mostly though, it's slightly easier to expel those from your main accomodation.

But I think in any case your ownership of the property is not challenged. It's "just" that you cannot use it, but eventually you can get it back.

But a notary isn't involved in renting, so there was noone to check.