At least you have some volume to work with. While not optimal, it can be dealt with - provided you take some care with where you put the mix position.
The rectangular room will be easier but also seriously restricts the volume of the space - no free lunch...
The other option is to have it broken into 2 pieces - 1 for mixing and 1 for tracking - though the tracking would be a bit odd and fairly small. However, IIRC from another thread, you're doing primarily vocals - which can tolerate a smaller space and benefits from being REALLY dead.
Bryan