I can almost always tell when I am being intentionally insulted. I NEVER thought of the term 'Mur'kin or Mer'can...whatever... as anything but a sort of Southern or rural pronunciation, and/or as a way to poke gentle fun AT them or at some aspect of being in that culture.
I NEVER associated the term with 'merkin', the pubic wig.

In the same way, I have NEVER used 'Brit' in an insulting way, but merely as shorthand when I meant 'all you folks over in that batch of islands'.

What I do is assume that I am NOT being insulted, until it is clear that I am. It is a LOT of work to keep track of ALL the little things someone might object to. I know the really offensive ones, and avoid them.