My answer to your question about how does the flash in the pan light the powder is, when you pour your loose powder down the barrel it will cover the end of the touch hole liner.
When you ram the bullet/patched ball down the bore all of the air under it will be blown out of the touch hole, carrying the loose black powder with it into the liners cavity.
When the powder reaches the smaller, actual vent hole most if not all of it will be blocked by the small size of the hole.
That puts the powder about 1/16 inch from the outside of the barrel.
At that distance, the flash of the priming can't help but light the powder in the vent liner and barrel.
As for cleaning, if your guns barrel is easily removable, take it off of the stock.
Put the breech end in a bucket of water. I like to add a little dishwashing soap so it will wash off any of the patch lube that's in the bore too.
With a cleaning patch on the jag, run it down the barrel to the breech.
When you pull it back up it will suck a LOT of water thru the touch hole into the breech and the bore.
Then, force the patched cleaning jag back down the barrel.
This will create a blast of water thru the area that will totally blow away any fouling that has collected in the guns breech.
After several of these pumping actions, the barrel will be clean and must be dried by running dry patches down the bore.
WD 40 can drive out any extra water down in the breech but I usually hold the breech of my guns barrels over the gas kitchen stove until I hear (and see) the water at the breech boiling off.
Application of a good gun oil to the bore and breech finishes the job.
About scratching your barrel key (wedge): Don't worry about it. It is made to get scratched up when it's doing its job like it should.
Do NOT use plastic wood or anything like it to "fix" your guns stock or the fit of its parts.
That stuff is ugly. It doesn't take a stain and it is too weak to survive the rigors of life on a rifle.