Mate, the cavalry attacked from the flank because the opposition was fully distracted by ONE person, Ike. Each using a pistol once to break through their lines, Ike who you just said could do amazing stuff with magic, so obviously there is no chance of friendly fire.
Completely different with a melee between armies. Of course there are still people giving commands and oversee the fight. Examples of foolish/cruel/ruthless commanders in different series (incl. this one) are typically shown by them firing ranged weapons into a melee because it leads to friendly fire.
Anyway, all that above are not very relevant to the discussion.
What’s important is the original argument, which you just responded with some handwaving:
It isn’t just prepping, Ike can’t produce fuse, gunpowder, bullets, or find and service guns with magic. There was an arc about this. So even if he can dry them, he need to source them. All those in a very short time.
If you say he got someone to find the gunpowder, fuse, bullets, and guns in advance, at that point why not make them prep the guns as well.
If you want me to concede the point, instead of debating tangential points, refute the original argument.
Perhaps there is a later or side story in the original media that shows Ike can now transmute gunpowder or bullet? Or one that shows he can fix/refresh guns somehow? Or even a later revelation that he exactly do what you said in the original media.
All you got to do is show in any part of the story: manga, novel, or side story showing something which directly refutes this statement: “Ike is unable to produce and prepare gunpowder, fuse, bullets, muskets and pistols with magic”.
