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"
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.
Thank you.
Coleman