But would you consider that a combat death? True the Bombardment was over and ceremony was supposed to be a part of the surrender, therefore it was supposed to be civil. BUT as with any accident investigation there are always causes. Again these were supposed to be trained artillerymen. And from what you say may have been the cause of the explosion comes down to trying to fire too fast combined with poor training, in failing to swab properly, or poor judgement in trying to fire too rapidly and if the still burning fires were a part of the chain of events.
I would contend that the poor judgement was a result of the situation. Maybe either Major Anderson or the battery commander trying to "Show Off" before their enemy. That seems evident in a 100 gun salute. Why not just a 21 gun salute for example? After all Major Anderson's garrison was not large enough to fully man very many of those guns (i.e. the shell being stacked too close to the guns and being set off as secondary explosions) if very many of them (the guns) were still in servicable condition after the Bombardment.
Firing a 100 gun salute would seem a bit excessive under those conditions. They certainly were violating what we would today consider to be the acceptable safety protocols in order to fire this salute. I realize that then safety wasn't a big concern, but by stacking shells too close to their guns, to speed up the reload time, would seem to indicate they were evidently hurrying their shots with too few men from what you say.
In other words in order to put on a display, the battery commander put his gun crews under combat conditions in order to impress his enemies resulting in a numbers of casualities including two unnecessary deaths.