A PC would be way overboard and out of its natural habitat for a morning church service.

Given your options, I would go with:

MacMillan tank (obviously)
No jacket or vest
Plain black belt with thistle buckle
Dark green long-sleeved collared business shirt (Claret, maybe, as 2nd choice)
Brown hose. Give them a chance!
Solid black tie (though it would be better if you could find a coloured one; got any striped ones that will work?)
No other options on flashes besides tartan ones?
Black Oxford shoes
Black leather "semi-dress" sporran

From the description of your church, it sounds like smart casual is called for. Not too dressy. I'd stay away from too casual as well, even if it's OK for others. Don't wanna go too casual for a first impression with the kilt.