If I made a custom with a tuning not in the game (Drob B for instance) would I be able to still play it and would the game let me tune to Drob B in game??

yes it will, it just won't show the tuning name and simply indicate "custom tuning", the sorting by tuning still work since it only used the difference from E standard and not the name.

Oh cool. Good to know. Thank you very much.