Have the house/property appraised in case of inheritance or gift

You are sliding into the realm of the Criminal Code with this... just mentioned in passing. Even if you don't want to admit it: some subordinate offense of section 263 applies there, e.g. deception. But you yourself say:

Zu sehr "zaubern" wollen wir da nicht,

No, it really should be legal there. We don't want to cheat, after all, the question first is how the property is valued.

And if mother-in-law wants to transfer her little house "early," she will get the right of residence anyway, one way or another. That is absolutely certain.

If relevant, also keep in mind that in the case of a gift with a residential usufruct right, the compulsory portion for heirs does not disappear linearly over 10 years but remains fully intact. Under certain circumstances, this can even be disadvantageous for the tax exemptions of the donee and then the heirs.