To what extent can Deshadharma make exceptions to Sashtras?

As per Baudhayana: “They are, to eat in the company of an uninitiated person, to eat in the company of one's wife, to eat stale food, to marry the daughter of a maternal uncle or of a paternal aunt.[2] 4. Now (the customs peculiar) to the north are, to deal in wool, to drink rum, to sell animals that have teeth in the upper and in the lower jaws, to follow the trade of arms, to go to sea.[3] 5. He who follows (these practices) in any other country than where they prevail, commits sin.[4] 6. For each (of these customs) the (rule of the) country should be (considered) the authority. 7. Gautama declares that that is false.[5] 8. And one should not take heed of either (set of practices) because they are opposed to the tradition of the Śiṣṭas.” Here Baudhayana seemingly allows Deshadharma or the Achara of specific countries to override the Smritis, though he cites Gautama to give an opposing view. Narada Smriti 1.40 says this: “When it is impossible to act up to the precepts of sacred law, it becomes necessary tot, adopt a method founded on reasoning, because custom decides everything and overrules the sacred law” South Indians would cite Deshadharma to allow for cross cousin marriages though it is against the Smritis and Shrutis. Now obviously there has to be a limit in which Deshadharma or Achara can make exceptions to Smritis. Like, what if a particular Hindu region so happened to eat cows (outside of ritual sacrifices) and cites Deshadharma to justify this mahapataka? So to what extent can Deshadharma and Achara make exceptions to general Smriti rules.