In what conditions should a pronoun use forms of singular they? [duplicate]

To say if there is a semantic error, we would need to know what the intended meaning of the sentences is.

“Someone parked their cars at the entrance” is incorrect if it is meant to convey that there was a single car parked at the entrance, but correct if it is meant to express that multiple cars were parked at the entrance. A verb used with we, you, or they as the subject will be inflected to agree with the pronoun, but nouns are never inflected into the plural simply to show agreement with another word. “Their car” and “their cars” are both grammatical, but with different meanings.

“The scientist dedicated themselves to the research” is different because the “-selves” in “themselves” is not an independent noun, but just part of a pronoun. “Themselves” can be used to refer to a single person.