What exactly constitutes a TD when the running takes a dive in to the endzone? Does the ball have to touch the turf in the end zone?
The ruling on the field was that Lynch broke the plane of the endzone per Rule 11, Section 2, Article 1a of the NFL rulebook:
A touchdown is scored when: the ball is on, above, or behind the plane of the opponents’ goal line and is in possession of a runner who has
advanced from the field of play;
Lynch scored the TD because the officials ruled that the ball was on the Falcons' goal line before he fumbled. Additionally, the rulebook notes
The ball is automatically dead when it is in legal possession of a player and is on, above, or behind the opponent’s
goal line.
The rulebook also sites this case specifically as an example:
Third-and-goal on B2. Runner A1 goes to the goal line with the ball over the plane of the goal line. He is tackled, fumbles,
and the defensive team recovers in the end zone.
Ruling: Touchdown. The ball is automatically dead at the instant of legal player possession on the opponent’s goal line.
Because the ball was dead immediately when Lynch scored the TD by breaking the plane of the endzone, the fumble could not happen. The play was over the instant he scored.
Why was the TD reviewed?
As for why the play was reviewed, all scoring plays and fumbles in the last two minutes of each half are always reviewed, outlined in Rule 15, Section 9 of the rule book:
After all scoring plays, interceptions, fumbles... after the two-minute warning each half, and throughout any overtime period, any Replay Review will be initiated by a Replay Official from a Replay Booth comparable to the location of the coaches' booth or Press Box.
Whether it was initially ruled a TD or a fumble, it was an automatic review by the Replay Assistant in the booth.
What would have changed if the ball was fumbled and recovered?
Rule 8, Section 7, Article 6 of the rulebook states
If a fumble by either team occurs after the two-minute warning: (b) The player who fumbled is the only player of his team who is permitted to recover and advance the ball. (c) If the recovery or catch is by a teammate of the player who fumbled, the ball is dead, and the spot of the next snap is the spot of the fumble, or the spot of the recovery if the spot of the recovery is behind the spot of the fumble.
If it had been ruled a fumble, the Seahawks would have had possession of the ball at the 1 yard line, where the center recovered the ball. In this case, either Lynch broke the plane of the endzone and scored, or he fumbled and it was recovered by his teammate, giving them the ball at the 1 yard line and another chance to score.
