Mike Z is our finishing expert and he chimed in here. I suspect he will hop back on when time and discuss further. This is a very interesting discussion to say the least. I will back away and let the experts in finishing work this one out.I am now suspect of the shellac, that it is not drying or curing properly, but my suggestion is to get in touch with Colman @woodworkingshop.com and he or one of his guys should be able to help.
I am going to "jump-in" on the "poly as a sealer" idea, I have NEVER had success with that, it seems like it is "pasticy" and loads a disc faster than anything! YMMV but it would NOT be my "go-to"