Yes, the high salinity could have been one of the factors in your clown's death. I do not know if clowns can happy live at that level of salinity, but from what I have read they should be fine. I am sure they cannot happily go from the 1.019 - 1.021 that your LFS was likely keeping him at to 1.028 without proper acclimation over days. Add the stress/poisoning from the cycle and 100% water change with fresh mixed salt and I think you had a doomed fish.
I say this only being two months further along the learning curve than you, but I can tell you I have learned a lot in those two months. Sadly, a beautiful little clown had to die for my education as well. I know mortality is part of the hobby, but I genuinely hope that he was the last one I will kill because I made a mistake or was impatient. I thought the hardest part would be waiting for the tank to cycle, but that was easy for me. The hardest thing I have found is not taking impulsive actions when I think something is not right. I have had to implement a hand in the tank at most one a day rule and just leave my poor fish and other critters alone.
You are doing the right thing by posting hear and getting advice. So in short learn and don't give up.