I will try to answer your question. I hope this helps you. I'm don't have the golden fingers explaining things I do like some on here.
I also mostly thumb seat my bullets but I do use a slight taper crimp just tight enough to hold the bullet. I don't use a roll crimp. My bullets are snug inside a fired unsized case but they will drop out if I turn the shell bullet down with a slight shake. These bullets are PP, I don't shoot grooved bullets except for a hunting load in my 95 Marlin or the .444.
Thumb seating a loose bullet can give you a problem especially with a soft alloy if that loose bullet or even a bullet taper crimped to hold it in the case if that bullet is seated with a lot of runout. I have recovered bullets I wanted to see just exactly what a bullet seated with a lot of runout cast with a soft alloy of 1/30 T/Land several bullets had a bent ogive enough that the land cuts were almost up to the ball radius nose and on the base on one side no land cuts.
A PP bullet that is seated farther than a GG is more forgiving because even a patched bullet a thousand or two will mostly be inline pretty close to the bore.
There are several ways to control fouling. You can do it like the guy in the photo below
or use what ever is best for you to get the bore clean between shots fired. There are several different things being used but for me shooting my unloved PP bullets I use one or two damp patches with just plain water.
Some use an oil/water or what ever concoction in the mix or some sort of bore pigs that has felt wads fixed on a bore brush and some sort of O ring at the end to wile the excess moisture. They all work.
And there is another method I use quite often when I just go to the range shooting iron or bowling pins hanging at 200 yards. This is using a proper designed bullet that will allow me to shoot dirty, no blow tube or wiping between shots fired.
The target I posted on post 13 is a 100 round target I shot with out using a blow tube or wet/dry patch it was mostly controlled like the feller on the horse
except I blow now and then into the breach through my cupped hand when the shel gets a little tight. That rifle had a 35" long barrel and that group is a 2MOA except for 3-4 shots at 200 yards. The caliber was a .44-100 Rem straight with a 500 gr bullet so the couple shots that went outside the group could have been me getting pounded with the heavy load during the 100 shots fired
Breach seating is done with a tool that pushes a bullet into the throat at just in front of the loaded shell, Some will seat it deeper but I seat mine just in front of the shell with a full case of black and a wad flush seated at the case mouth. I patch the bullet one or two thousands over groove diameter also some a little over bore diameter depending the bullet profile I'm using.
The biggest issue I find with vertical is the loss of pressure behind the bullet. This happens from an improper bullet diameter and wad stack mostly but there are other factors. When the gas gets by a bullet before it expands when the powder gets lit you will see this on the paper or chronograph. It's in my opinion the biggest issue with verticals.And also recovered bullets from the snow piles will show what is going on with gas cuts.
But also powder volume and a bunch of other things going on.