No matter what else happens in this scenario, the Cowboys would be the NFC East champions. They would be no worse than 9-7 and no other team would be able to win more than eight. If the Cowboys beat the Giants on Sunday and again on New Year's Day, it doesn't matter what happens in Weeks 15 and 16.
Giants beat Cowboys twice and beat EITHER the Redskins or Jets
If this happens, the Giants will be 9-7 and the Cowboys would be 9-7 at best. Even if the Cowboys win their other two games against Tampa Bay and Philadelphia, the Giants would be division champs by virtue of the head-to-head record.
Giants beat Cowboys twice, lose to Redskins and Jets; Cowboys win their other two games
The Cowboys would win the division outright with a 9-7 record, as the Giants would finish 8-8. However...
Giants beat Cowboys twice, lose to Redskins and Jets; Cowboys go 1-1 in other two games
The Giants and Cowboys would both finish 8-8, which means the Giants would win the division by virtue of the head-to-head tiebreaker as long as neither the Eagles nor Redskins finish 4-0.
Giants and Cowboys split head-to-head games; Cowboys beat Buccaneers and Eagles
The Cowboys would be 10-6 and an outright division champ.
Giants and Cowboys split head-to-head games; Cowboys beat Eagles.
In this scenario, it doesn't matter what the Giants do in their other two games or what the Cowboys do against the Bucs. The Cowboys would be 9-7 and would hold the tiebreaker over the Giants due to a better conference record.
Eagles finish 4-0, Dallas finishes 1-3 or worse and Giants finish 2-2 or worse.
The Eagles, Cowboys and Giants would finish in a three-way tie and the Eagles would be division champs by virtue of tiebreakers. They'd be 3-1 in games against Dallas and New York. The Cowboys would be no better than 1-3 in games against New York and Philadelphia. And the Giants would be no better than 3-1 in games against Philadelphia and Dallas. The second tiebreaker is division record, which would go easily to the Eagles at 5-1. If the Eagles finish 4-0, Dallas finishes 1-3 and the Giants finish 1-3, it's a two-way tie between the Eagles and Cowboys, and the Eagles will have been 2-0 against Dallas, so they'd win on the first tiebreaker. Note: This happens even if both of the Giants' victories are against the Cowboys.
Redskins finish 4-0, Cowboys finish 0-4, Giants lose to Jets on Christmas Eve
The Redskins and Giants would finish in a tie for the division title at 8-8, with the Cowboys 7-9 and the Eagles also under .500. The Redskins would claim the division title by virtue of having won both games against the Giants. Also, they'll have finished 4-0 without two of their best offensive players, Trent Williams and Fred Davis, who are suspended for the rest of the season for drug policy violations. I'll have to look up whether those guys would be allowed to play playoff games.