I'd go with using the stall....and move the run next to the barn, instead of using a tunnel(which might be an impediment to 'traffic' in the area).
PITA to have to 'rebuild' but better to just get it over with.
That small coop is not a loss at all, it looks well built and will come in mighty...
Depending on your climate (putting your location in your profile can help folks give better suggestions) 4x6 is good for about maybe 6 hens.
Way too small for 15.