Well, I hate to drag this on. following Randy's common sense reminder as to how DC locos were wired, (my reference is Athearn BB locos) , and have working directional lights, maybe, if I were the OP, I'd consider disconecting the speaker, and leaving the DM decoder in place, which would seem to be the easiest route for the lighting.
I have no idea how any of this would affect the plug-ins, between loco and tender.
Like I said, my experience with any of this is older Athearn BB locos, that did not have the board.
Mike.