Unfortunately seems like most likely a combination of things, mostly being stress, ammonia and sg issues. My lfs packs most their fish in 1.019 and most fish I acclimate in the bag for less than an hour. I had 2 anthias I acclimated in the sump just as you described and I raised sg over the span of around 2 hrs. Both fish did fine, I used a chip clip to keep the bag nearly sealed. This sounds like the experience your more accustomed to and seems the 2 hr drive home is the outlying difference. Imo stress of the animal along with very possible ammonia spike then adding the salinity change on top of that prolly was just too much too fast for the fish. When I bring fish home they usually go from store to home in 15mins or less.
I would think an lfs with low salinity would be either giving out store credits like crazy for dead fish or making sure customers are aware of proper acclimation for their fish considering. But going forward having a qt and matching lfs salinity sounds like the best way to cover bases. And helps me, if I ever get my new tank in the mail plan to have a one spot and know I shouldn't be leaving in the bag now

sorry about your fish tho, that's tough