Yulzari, welcome!! and thank you for the very much spot on summary of the situation with the Mexican army at the Alamo (and for that matter afterwards as well) with arms and powder. as you have well noted, there was nothing wrong with the India pattern Brown Bess muskets, nor the Baker rifles.
From one source I read after the end of the Napoleonic campaigns, in what was noted as pretty much standard British fashion, the 'war is over' so time to unload the surplus.
These arms were simply in excess for the now peaceful status, so perfectly functional.
As noted, the 'training' of the Mexican army ranks was inadequate and deplorable, just as the supply of required materials such as powder. The corruption notoriously known in the Mexican government especially under Santa Anna certainly led to the poor production leading to miserable powder.
One thing I was trying to discover here was where in the world did Mexico in these years get the powder they were using? As noted, the British had very good quality powder with their considerable sources for saltpeter, however I feel sure the Mexicans thought they did not need that expense and could make their own. In all the resources I have checked there were mentions of certain arms works, but no mention at all of powder production.
With the Texians, although in limited supply, the powder they had was apparently Dupont, which was excellent powder, but diminished in quantity.
With all the powder however, the elements of weather and dampness rendering it relatively inert came very much in play.
Thank you again for joining us here! and never worry about reiterating something already said
These threads can get pretty long and intense and often its good to bring up certain aspects that are key in the discussion from time to time.