is it possible to return a new but opened game?

I think you can return it as long as it is within the return deadline (usually like 30 days or whatever) and if it is not damaged. Ideally, you should try to make it look like you never opened it, but that may be difficult. Also, it really helps to have a receipt. Anyways, store credit or an exchange are usually the most likely options. So, basically, yes, you should be able to return it.

It depends on the store policies - some stores will take it back before the return date, even if opened, some won't because it's been opened. You should check the back of the reciept and see if it says specifically anything about the electronics department. Also, if they don't take it back, you can always sell it to a store such as Gamestop.

If it's before the return date you should be fine. if its after the date you can probably piss them off enough for them to give you store credit.