A855 - 5 Speed

Make a log the issue. For the log, list your actions and their responses and promises in chronological order. Code the items so it is obvious for the items that you have undeniable evidence of, for example, dated e mails, videos, photos, checks, credit card statements..
also list:
.money spent
.time invested installing & removing the transmission
.time your car has been unavailable to you due to transmission issue
. What you want, your money back and compensated for your time invested or another transmission

take that to a lawyer, pay them 200.00 to type it up and send a copy via registered/certified mail where proof of delivery is obtained. Don’t get the response you want, take it to court. Passion will need to travel to your area for any litigation activity, so that is in your favor concerning getting them to agree to a reasonable refund.
You just need to let them know you are serious and are not just willing to let this go away.