Before this past week, if Seattle won only is 4 remaining home games and lost its last two away games, @ SF and @ NY Giants, they would win Home-field Advantage throughout.
If the Saints finish with the same record as the Seahawks but having lost to in Seattle; Seattle would own the tie-breaker over New Orleans in head-to-head.
If the Niners had won against the Saints and won out and ended up tied with the Seahawks, they would be tied head-to-head 1-1, tied in the division 5-1 and it would fall to common opponents... which would mean that the Seahawks would have won because they would have been 4-0 vs. the NFC South vs. the Niners 3-1 (assuming they beat the Saints) with them losing to the Panthers the previous week.
Now, the Seahawks need to win 3 home games and they get HFA over the Saints. The NFC West Division Title is down to a magic number of 3.