There is one other possibility which might be easy but ugly-I could
run the pipes above the floor, along the inside of the outside wall
(this would be behind the toilet) and then under the tub to its
faucet, but this would leave about 2' of pipe just dangling out
there. This doesnt bother me but it might bother a potential buyer...
Based on everything else you have written so far, I think you should
probably just go ahead and do that. It seems like you really want the pipes
on the inside and accessible, and that would solve both of those issues. A
pipe along the wall, near the floor, behind the toilet seems like nothing to
me. You could paint the pipe(s) the same color as the wall or baseboard and
it will look fine. You said, "under the tub" -- do you mean under an
old-fashioned claw foot type of tub?
As far as buyers, I doubt that 2 feet of pipe going along the wall near the
floor behind the toilet is going to make or break a sale. And, since the
temp goes down to 40 below in your area, some buyers may like the idea that
the pipe will be inside and accessibole for the same reasons that you do.
And, if you are really concerned about buyers, you really need to figure out
a way to get rid of the mice and rodent droppings that are infesting the
area that you don't want to go into. A home inspector will find that huge
(in my opinion) problem if that area has an access door where he/she can
enter or look into the space.