Can I submit another version?
You may reedit your submission post if you want to replace it with a new/redrawn one, but you cannot have more than 1 submission total.
This is silly and not in the competition's aim in my opinion-it's all about 1st try 1st go...
Well, there's nothing wrong with practicing, IMHO - if you feel that practicing produces the most fun and best 5-minute attempt you make in the end, then feel free to go ahead and do that. One of the main points of this competition is to have fun, though, so try not to stress too much.