hicetnunc, those are difficult questions to answer, and I will not even try. I've only looked at the data for the first White move, so that's all I can say anything about at this moment.
NM Reb, I agree that the quality of the games in the database does influence the results. Still, they do tell us something. Like, do play 1. c4, don't play 1. f4.
It would be interesting to look at the numbers for a million games played between chess engines rated above 3,000, where White is forced to play each and every of the 20 possible opening moves 50,000 times.
It is highly unlikely that some first moved will not turn out to score better than others.
There is no "best" first move for white and relying on statistics compiled from databases that also include very questionable games is a sure way to be led astray.