With this evening's San Diego win, my understanding is that with a win tomorrow, they clinch the division (on account of a 12-5 record against San Fran), UNLESS...
Atlanta wins Sunday (I'm assuming a loss tonight).
If San Diego wins & Atlanta loses, San Diego wins the division with San Francisco getting the wildcard. This means San Fran would travel to Philadelphia and Cincinnati would go to San Diego.
However, if Atlanta wins, it forces a 3-way tie, which means San Diego and San Francisco are not guaranteed of both making the playoffs, thereby scrapping the head-to-head record.
My understanding, then, is there would be two (2) one-game playoffs to settle the wildcard & West division. Perhaps someone can clarify how that goes down.
If Atlanta wins and San Francisco wins, Cincinnati will play Philadelphia.
If Atlanta loses and San Francisco wins, Atlanta and San Diego will have a 1-game playoff for the wildcard. Atlanta winning would have the Reds facing Philadelphia. San Diego winning would have them playing San Francisco.