Thank you, if the finish hasn't dried in say a month from now I will try that.
Thing is.. the tin says, do not cut/thin.
So I may need to just opt for a 'fresh' / in date can of the same Amber Shellac.
But that technique may well do the trick, given the shellac doesn't really 'layer/coat' like a paint and instead merges making one thick coat.

My understanding is that the Zinssers Bullseye Amber Shellac product contains Wax and for that, or other, reasons cannot be thinned.
Their Clear shellac can be thinned, but I suspect if I were to use a thinned clear over the amber I may get a reaction in the finish and make things worse.

So for now I will play it safe.. and 'stick with the current finish' in the hope it drys in time.