Sentence "Grób myje się" remains incorrect though (or causes ambiguity in meaning) because " grób" appears to be an agent here, not the patient, its meaning is basically equal to: "Grób myje siebie".
Why do you think that polish grammar is disambiguous? I agree that this alone looks very odd, however imagine putting it (a little re-phrased) in context like this:
a. przepraszam, czy można?
b. proszę nie przeszkadzać, tu się teraz pracuje!
a. tak? a co dokładnie się robi?
b. tu sie teraz myje grób.
you still say that these sentences are grammatically incorrect or ambiguous?
Of course, I could imagine dialog like this only in a Bareja movie ore something like that, but I'm sure every Pole would understand it perfectly.
Cinek
Using inanimate nouns certainly requires more attention, but doesn't make using them in this type sentences incorrect per se.
It's exactly what I wanted to say.
"grób' in 'Grób myje się' very distinctively takes the accusative case, whereas "grób" in "Grób myje siebie" takes the nominative case
and thanks for formal support :-)
Cinek (którego czasem do końca się nie rozumie, choć stara sie jak umie ;-)