The leather I use is about 2mm thick. it is the back panel off an old leather chair. It is enough to grip the flint and the jaws on the cock. Some people prefer thin lead sheet. I have tried it and prefer leather.
I prime my flashpan from a flask after I have loaded it.
I think the best place to prime the pan is on the firing point. Once it is loaded and the flashpan has been primed it is in effect a loaded gun and should be treated accordingly.
That is just my opinion and others may differ.