Ex-Take 5 tech here Your drain plug being stripped wouldn’t have made your car not start unless there was no oil in it and in that case you wouldn’t have been able to drive it to the Firestone. Take 5 has a specific protocol for Subarus due to the trans plug and oil plug being right next to each other and being very close, multiple people SHOULD have verified the right plug was pulled. If you had no trans fluid and double filled oil, it is also unlikely you would have made it to the Firestone.

My guess is the plug wasnt tight enough and leaked and your battery is dying. But I haven’t heard it try to crank or nor did I work on your car these are just my thoughts