Honest question: does anyone here give a single shit about Mickey Mouse as a character or idea? Have you
ever? Because I can't name a single property featuring him that I've seen apart from that one
Get a Horse short which was pretty okay, I guess, but not because of anything to do with
him.
All of which has absolutely nothing to do with the story being told.
I agree: changing him to a disillusioned/washed-up Union Army veteran wouldn't have changed the story at all. So, it's all the more telling that Disney didn't consider his Confederate history worth modifying. Again, picture an unrepentant former Nazi officer in a similar role.
Him being a Confederate soldier had nothing to do with being pro-Slavery
There is zero context in which being a Confederate officer - not even a forced enlistee, mind, but an officer - has "nothing to do with being pro-Slavery."
As a fan of old pulp stories, I'm actually fairly tolerant of outdated and problematic elements and tropes. But, absent significant mitigating factors, of which there were none here, glorifying the Confederacy, even indirectly, crosses a red line of mine. My two cents.
(The movie was mediocre primarily for a hundred other reasons, to boot.)