Unhappily for some this is just further evidence that the stuff was made to be drunk (with water sometimes ) and the packaging was not designed for long term storage. I'll bet even the Aberlour a'bunadh with it's heavy wax seal leaks a little.
Drink them while you can because if you don't somebody else will and most likely with Coca Cola.
C_I wrote:(I just wonder, how to beef up a half evaporated Ardbeg from around 1994, to have still a bit of enjoyment from it, if ever possible)
Are you on good terms with the angels? They're the only ones who can help you. I would expect a half-evaporated bottle to be bloody awful, and beyond all hope. It's a total loss for the shopkeeper, so maybe you can convince him to open it and give you a sample.