I do not know if you are soliciting comments about your theory, but I think your interpretation of the stone inscription is great. I have just happened upon this Oak Island mystery in the last month...from what I can see, things don't add up. I have been reading non-stop about this and concluded that the access to the "treasure" does not lie in the original pit. The inscription in the stone tells in alphabetical cryptic where the treasure is...forty feet below, but it also tells you using the cryptic symbols how to get there, enter some sort of tunnel/cave system at another point. Your assertation that this system reaching all the way to Birch Island is pretty awesome. I think the original hole was designed to hold the instructions, but then thwart entry into the caves/tomb/chests below and force the diggers to look in the right place (Birch Island) for the entry point. I don't think the original construction could foresee steel drilling equipment, dynamite, hydraulic pumps and excavating machines trying to access the caves etc. from above at the site of the original money pit. They probably thought the water would be sufficient to force people away...who can hold back the ocean? Except when it freezes!
There are a lot of theories about the nature of the "treasure". There was one researcher who presented an idea from a work by Bacon which suggests that the answer lies in the "hail and snow" and to look to the light that ends the darkness. When it freezes, there are four peculiar round formations in the ice off the island and Birch Island lies to the east where the sun rises...? There is also a wreck between the two islands. I think the holes in the ice are important, either triangulation points or evidence of an underground tunnel system which perhaps releases warmer water from a spring. The triangle and round spot on Birch Island looks a lot like the grail symbol explained in The Da Vinci Code...I know that is a little crazy, but the "treasure" hunt is continuing in the wrong place. The triangular clearing on Birch island is not a natural formation by any stretch, and would not happen coincidentally by someone just clearing trees for wood.
I think the money-hungry fools who have been digging and drilling and blasting for 200 years are just making a mess of things. Hopefully, whatever "treasure" lies buried is still relatively intact and accessible. Hopefully, the drills didn't perforate the oak casket and gold adornments and the body of someone important to history, or crush the 'Holy Grail' itself!! Or, allow mud and seawater to enter a sealed cave sytem, ruining any artifacts which may have been preserved there. I think you have taken the necessary step back and looked at things with fresh eyes, hopefully someone listens. I will continue to follow this story, and I invite you to reply if you are interested.
Neal in Victoria BC.