But you want one bottle succesfully opened, so it's perfective. That should usually be a quick action, so imperfective 'otwieraj' would feel quite strange.
"Otwieraj!" may be used when you tell someone to open the door, but that sounds rather rude, unless you have a very good reason to 'command' someone in such a way. For example if you're being chased by a dog. Okay, I guess it may also work with a bottle, but again, that seems as if you're in a hurry. Also, it would feel strange to me with this "mi" part. If I hear "Otwieraj butelkę!" it sounds like "Dude, hurry up, let's start drinking finally".
That's a good question. If it's any similar to Russian, then the imperfective is more immediate or urgent, but either the imperfective or perfective can be used. For example, otkroj butylku means "open the bottle" in general; otkrywaj butylku means the same thing but it's a stronger command than the former, meaning "open it now"