NFC west isn't getting 3 teams. There's too many games between them all in the next 5 weeks. So Detroit imploding would almost guarantee 2 from the NFC East
If Dallas sweeps (and thus beats the Eagles twice), they'll win the division.
If Dallas loses once (and thus beats the Eagles once), they'll either win the division, beat the Lions outright for the wild card, or beat the Packers in tie breakers.
Dallas only has a problem if they have two or more losses. In that scenario it's better to have tie breakers against the Seahawks (who they beat) than anyone else. And even then its only a problem if it's a 3 way tie at 11-5 between the Cowboys, Lions and 49ers/Cardinals. In that scenario, the Lions would advance based on conference record and the 49ers/Cardinals would win the other spot by virtue of beating the Cowboys earlier this year. Thus its to the Cowboys advantage to root against the Niners so that they'll face the Seahawks in a 3-way tie instead.
If Dallas loses 3 or more games... Yeah, Dallas better be rooting for a Lions collapse. But in this scenario, 3 NFC west teams making it to the playoffs actually is plausible so the Cowboys are probably fucked anyway.
So yeah, rooting for a Lions collapse is only necessary if the Cowboys collapse too. Otherwise its the 49ers who are their worst enemy (outside Philly of course)