Congratulations on your dark brown congnac McTavish - I am picking mine up today.
I think the black McTavish would be fine as it sounds like you dress more business / casual. I think the black McTavish will work with khakis and chinos - but only if those pants are navy, black, dark gray, etc. I personally don't like the tan / light tan slacks and black shoe look - but that is what the congnac McTavish are for. If you don't wear a lot of navy, black, gray chinos and khakis, I would just go with the Cognac McTavish.
The suit thing complicates it a bit. I don't feel the McTavish works with a suit. But if you don't wear your suit very often (sounds like maybe 1-3 times per year) a pair of McAllisters might be a bit much for such limited wear. And I don't know if the McAllister will work with more casual pants as the McAllister is a balmoral.
If you want to get a black shoe that can do a suit and you can still dress it down a bit, I am going to suggest the Kenilworth in black. The plain toe can go either way and the blucher will let it dress down a bit.
Good luck in your search.
A very decent post.
The McTavish is too casual for a suit. The McAllister is too formal for jeans, and is also too formal, IMO, for khakis or chinos. A ptb is a good option.