In European Airport Duty Free Shops, can Europeans buy Duty Free?

it's nothing to do with your nationality or your passport, it's about where you are travelling from and to. You, whether American or not, can shop duty free in Europe only if you are travelling somewhere outside Europe. No-one can buy duty-free when travelling within Europe. You can, however, of course buy the local products at the prevailing price, which can vary between European countries. For instance, if I were flying to the UK from Greece I could not bring in duty-free cigarettes but I could bring ones i had bought in a shop in Greece (duty-paid), which are still cheaper than they are in the UK.

Duty free is not about where you are from. You are only allowed to buy something duty free if you are traveling OUT of a country. If you are visiting Europe, when you are on your way back, they will check that you have a ticket to leave the country. Then you and your husband will be able to buy things in the duty free shop.

You should be able to buy alcohol in London, but it will be taxed just as if it was bought in a shop outside the airport. When travelling within the EU* there is no duty or import tax to be payed, as long as the alcohol is for non-commercial use. On the other hand you will not be exempt from any taxes when buying the goods. The situation is similar to when taking alcohol over state borders within the USA. *Some areas, such Åland and Martique, are outside the EU value-added tax area, while certain areas outside the EU, like Monaco are inside.

Yes you can. You have to only show a boarding pass for outbound international flight. You have to be carefull with buying liquids.