You probably could but I wouldn't. Why water down a perfectly good beer? I'd rather have 45 good brews than 50 watered down brews. Just bottle what you got.
I would not. You are adding potential contaminates/infections. I don't think you are at a huge risk for this but it's not a very good practice. More importantly you will be diluting the beer down. Was the OG too high to begin with?
If you decide to do it at least boil the water first. This would greatly reduce your chance of infection.