I agree with all has been said so far here. Your UI is basically what Tony proposed and what I would like too.
Yes, there's no need to delay both, that also seems to make little sense to me, it is the resulting difference in the two hypothetical delays that would matter, so there's no point in dalaying both.
As to the distance, I have no answer to what you ask. Your point is meaningful, I believe, unless the shape of the car interior could require delay values that are different from what you would simply measure by considering the distance between the driver and the speakers. But I am really just guessing, here...
And, sorry for not saying this before, a big big THANK YOU for your work!!