Financed Subaru Forester 2.5i with dead CVT transmission

I have a 2014 Subaru Forester 2.5i.  I have changed the oil on it before, but
when I did it this last time, I accidentally drained the transmission fluid (the
two plugs are only a few inches apart) and not realizing my mistake, put my
5 quarts of oil in and backed the car off the ramps. The next morning, I went
to drive, and made it about 20 feet down the driveway when all the lights
came on.  I tried backing up but had no transmission power.  I coasted down
the hill to a place to park.  Realizing what I had done, I had the car towed to
a shop.  They filled/serviced the transmission, drained and properly filled the
oil, but on test drive, the transmission is shuddering hard and not shifting
gears.

My problem is that I currently owe about $12k on the vehicle still (I am under
water on the loan) and the dealership is quoting $8-9k for new CVT.  I have a
shop that quoted me $2400 for labor to install a used CVT, but still looking at
a total of around $5-6k, depending on the cost of used CVT that can be
found.

It has 135k miles on it and KBB is about $7k.  

It seems stupid to sink almost $6k for a used CVT into a car that is only
worth $7k (knowing that the used CVT may vary well have the notorious
problems of CVT transmissions as well) and even more foolish to put a new
CVT in at the dealership.

I'm mad that Subaru would put the two plugs so close together, and pissed
that they made a transmission that is so delicate that it is trashed after
driving a few feet without fluid.  Yes, I know that is never good, but seriously?

I feel like I am royally up s*** creek on this situation, but I thought I would
reach out for any advice.