How much of the ejecta (length of shot and wad) has entered the cone at .0003?
I would say between 1/2" to 1"
Why is the pressure falling at .0005 with ejecta still in the cone?
Because it has traveled approximately about 2" at that point and is exiting the forcing cone. If you remember your fluid dymanics you would know that pressure drops when matter exist a nozzle. It also increases velocity, but good luck measuring that inside of a gun barrel. I wish we could.
What does overcoming inertia vs. friction vs. some degree of constriction on the ejecta upon entering the cone have to do with the pressure curve?
I don't know. My position is that it does to some, as of this point, unknown degree. As far as I can tell, your position is that it has no effect on it.
I said nothing about crimps, or wad seating pressure, or internal ballistics of the shotshell.
All of those things are not exclusive of this topic, nor can they be excluded when discussing peak pressures or time curves. My contention is they all have a factor to play, which sets shotshells apart from the muzzleloader.
Neither one of us KNOW the effect of the ejecta entering the forcing cone, nor can we answer (with numbers) the OP's question. Do I have that correct?
I'm not sure. You seem to be very disagreeable to the idea that it has any effect. I'm simply stating it must have some, which I don't think is a reach.
I know from reading your many posts over the years you seem to have made contacts with experts in this field over the years, and have access to testing equipment most of us do not. This might be an interesting experiment to take on if you are truly curious about it. However, I would agree the outcome of those tests would mostly be academic without much real world demand, i.e. pressure curves in muzzleloading vs breechloading shotguns . I think this one will most likely stay in the realm of theoretical debate, probably for good reason and not be worth the time or money.
Now coming up with experiments to test the effects of different forcing cone shapes/lengths in regards to breech pressures...that might be a different story
