Isn't some sort of wadding needed to both hold the powder in place prior to ignition, and to maintain pressure until the powder has substantially ignited?
Not for the most part.
The rifle is “loaded” in the upright position so the powder falls to the bottom. As long as the rife is carried upright, or at least the muzzle not dropped below the horizontal there is no problem with ignition.
If somebody does get the muzzle too low they might dump their powder or get a “phzzzzt” instead of a “bang”.
It is hard to “load” in this way during skirmishing so you’ll see most raise the barrel to a vertical position and tap the but on the ground to “seat” the powder.
The paper used to hold the powder can be rammed and is done so at some reenactments. For the most part there is nothing wrong with that but there are a few issues.
On the conn side some have been known to leave the rammer in and fire it, somebody might have used staples or such and they become a projectile and the paper is burning and does set grass fires.
On the plus side it keeps the powder in place, better ignition, slows the rate of fire to a more realistic rate and keeps the powder dry when it’s raining.
All in all, most mainstream reenactments go the route of most safety.