Point well taken.
But I would submit that the very salient points you make were only one very important portion of what the founders considered to be a 'well-regulated' militia.
They did expect that the militia would obtain proper basic firearms training within the sphere of their local leadership.
And there can be no doubt at all that they most certainly didn't mean 'regulated' in the 21st century meaining of that word.
I believe that in those days "basic firearms training" would have been hard to avoid, as youth literally grew up with firearms in hand.
Towns were mostly pretty small, most of the time every member of the local militia would have been neighbors and hunting partners.
This aspect was still common in W.W.I, particularly in England, which sadly led to the demise of the entire generation of young men from many rural towns.
Thoughts of Sheffield spring to mind.
I am sure the standards of the time varied widely, there would have been little central government oversight to ensure a single uniform expression of "Well Regulated".
I have seen scholarly articles asserting that in early America state and local government feared that the cost of making the militia too "regulated" would place an impossible burden on their treasury, as they would then be obliged to provide the militia with the required equipment!