“dhammā” needs to be understood in the context.
In “Katame dhammā anidassanaappaṭighā?” the question is asked: “WHAT are anidassanaappaṭighā?”
– That DOES NOT imply that “Nibbāna is a dhammā”
There are dhammā that come in “manañca paṭicca dhamme ca uppajjāti manoviññāṇaṃ.”
– On the way to Arahanthood one experiences various stages of magga phala via contact with the mind.
However, once an Arahant attains Parinibbana, there are no more “contacts with the mind” since that Arahant is not reborn with a hadaya vatthu.
Does that answer the question? May be I am not understanding the question.