If Arizona and Seattle both lose do we automatically get the second seed regardless of what Dallas does?
I think you are right.
With no GB/Det tie the North winner is 12-4 and top seed.
In the Seattle/AX both lose scenario Seattle takes the West at 11-5 based on head-to-head tiebreaker
If Dallas loses to Washington to also go 11-5 the only tie breaker in play is head-to-head vs Seattle, which Dallas won. So if both Seattle and AZ lose, we are the second seed automatically.
We still need to play hard in case GB/Det tie. Our game is first, before all of them. 11-4-1 still beats 11-5.