It's probably just a slight difference, but "Dzięki" is rather less formal ("thanks"), and "Thank you" is "Dziękuję" (a little bit more formal). That's how I see it
I think that thank you and thanks are different words in polish, Although maybe yeah thanks should be accepted
How is the 'e' with a curly actually pronounced? Polish friends of mine say it's just 'e' as in 'let' but Duolingo pronounces it 'ing' or 'en' as in 'sing' or 'pen'.
It is a nasalized "e". You can compare it for example to Spanish pronunciation of "euro" or "Europa". Generally, it's more or less "eu".
When it is the final sound of the word, many users (a majority?) pronounce it exactly or almost as a normal "e".
While many use it similar to a normal "e" when it's the final sound, are both ways correct and is it a regional thing?
From what I understand, it's considered wrong, although of course there may be regions where it's more common.
Depends on where it is in the word. If it's within the word, ~"en" (nasally "e") but when it's at the end of a word, use the standard english "e" sound.
Hmmm... it seems that it could often be interchangeable, but it would really depend on the context. Also 'dzięki' may be too informal for many situations.