I can get my hands around a few of your questions. Out of order, gas checks need to be placed on bullets that have a recess designed into the mould. If you took a flat base bullet and placed a gas cap over the top (if you could) and you could swedge it to the proper dia. you would greatly deform the bullet. The gas check can be applied to the bullet and run through a sizer as you know. If you use hornaday checks which have an inside edge (tooth) that bites into the bullet you can slip it onto the base of the unsized bullet and run it into the neck of the cartridge, not past the neck and it should stay put when fired. The neck draws it in against the bullet and the tooth helps hold it in place. if the check stays in place through the leade it will be well swedged on as it runs the barrel. By the by, the lyman checks do not have the burr as the hornaday have. I have just gotten some lymans and have not yet fired them unsized. One last point, I made up a jig out of a cut down case to operate my lee factory crimp die to make it a gas check swedge. It works fine. As far as sizing goes, the smallest you should size to is .001 over groove diamiter. As far as how large can you go, well you can be a couple of thou over and it will work out ok, Remember, if you do not size the bullets with the GC on it, do not run the base of the bullet past the neck. (bottle neck rifle cases) The GC could fall off and become projictile #2 going downrange. Not a good thing.... size.. gas checks run a bit large so you can get them on. I use 30 cal gas checks on my .312 lee bullets, they are snug to get on but they go on.... I don't know how you could open up checks other than make a punch to open them up.
Pistol bullets rarely use gas checks. You need them if you are going high velocity, high power. As for #6 I think you should work on your load and you should slug your barrel and cylinders to see exactly what you are working with. FYI Lyman makes a .41 cal GC mould. JB