Supermarkets, such as Tops, Big C, Lotus, Villa Market all accept AMEX, and most expat pubs will take it. I haven't found much else. I carry mine as I get extra points for using on airlines, so all my international and domestic flights (Thai Airways accept it) go onto it.
Yes but if you're referring to your passport in the context of Thailand, saying you have a "European" passport is worthless, as Thailand does not recognise a "European" passport. I have triple nationality, UK, Australian and German. All of these are treated equally by Thailand.
A lot of Europeans do not realise that Thailand treats every European country as a different entity. When people say "I've got a European Passport" I cackle, but there's actually no such thing. Each European country issues its own passport and Thailand treats that citizen on the merits of the home country