To answer your second question: Of course it would.
And, to answer your main question, I'm not positive. The winners of their divisions with the best two records host home games. The Seahawks and 12-3 and the 49ers are 11-4. If the Seahawks lose next week and the 49ers win, they'll have the same record. The winner in that case (if I'm not mistaken) is determined by whoever was better head to head. In this case, that's also tied, so it would be by the team that did better against division opponents. In this case, they'd be tied there too. I don't know the tie breaker after that.
However, NFL.com says Seattle has clinched a playoff spot, but it doesn't say they've clinched the division, so that tells me the 49ers can still win a division spot.