I don’t believe the embers in the bore scenario for a minute, unless you are shooting combustible cartridges. Yes, maybe a hundred and fifty years ago, when powder manufacturing was hit, and miss, you could end up with poorly ground,adulterated, or mixed, components that could leave an ember after combustion, but not today.
The reason most of us started blowing down the bore, was to make sure fowlings hadn’t backed up into the nipple, or touch hole, and plugged it up. It was simple, and easy, to simply blow down the bore, while watching the breech for the telltale puff of smoke from the nipple, or touch hole. We never dreamed that someone could mistake the pop of the cap, or flash of the pan, for true ignition, but some did.
I suspect that the majority of the issues in this area can be attributed to modern muzzleloaders have nipples, and touch holes with smaller holes than was common in the past.
Hungry Horse