Which of these rules for a Scrabble first turn play are official?
Like you said, the center tile counts as a Double Word Score tile, so whoever plays first gets double the points.  That is an official rule.

The second and third rules mentioned are house rules, and are not supported in the Scrabble rules.  The second rule would prevent you from hitting one of the adjacent Double Letter Score spaces, as even a 7 letter word wouldn't be long enough to reach it, since they are 5 spaces away.

Even the most recent Scrabble Tournament rules clearly indicate that the first word played must simply cover the center tile, and be spelled top-to-bottom or left-to-right.