Michl's thread on the position of the touch hole should answer sme of your questions.
http://www.vikingsword.com/vb/showth...ght=touch+hole
The development of the priming pan was, in my opinion, mostly due to the ever further developing gun. First guns are, as can be seen in contemporary illustrations, heavy objects. Lateron this changed to a more manageable hand held gun like the one in the Michael Trömner collection.
My guess would be that when these real hand held guns arrived, they soon discovered that a better "container" for holding the black powder was needed. Especiallyin the heat of the battle when you would often run, hide... aim. It was chaotic at best and no way that a powder load would stay neatly stacked on the touchole. As stated before, the thread on the position of the touch hole tells the same story. The priming pan slowly evolved from beeing just a ridge on top of the barrel to a pan situated mostly on the right hand side.
There are instances, if i remember corectly, that a hardening gunpowder past was put in the priming pan... but if this was yet discovered/used in the times of my tiller gun, i don't know.
All of the aboove is also just an assumption
I have no evidence that this thesis is correct but it sounds the most likely to me.