Can I use double (320px x 288px) resolution and use it as i want? Or I should scale it, so every pixel will be double sized (in this case)?

yes you can. i dont promise that this is what ive been told but yeah it makes sence because you cant play a game in thet resolution on a pc

You can (and should) scale the pixel resolution (2x or 3x recommended)

No to the first if I understand what you're asking correctly. That is if you're not upscaling to 320x288.

Yes to the second.

Just make sure that the base resolution that you're up-scaling from is 160x144.