Your commitment to playing out various scenarios is truly impressive. I'm too old, there are too many teams, etc. Let me try a different way, not even to prove a point...I'm just more used to this phrasing:
Let's say the Cubs go 6-9 (nice) to finish the year. Short of a complete collapse, kinda worst case scenario. Without considering head to head, and using UMFan's tiebreaker notes above, us missing the playoffs would require three of the following:
Phillies to go 5-11 or better
Diamondbacks to go 8-6
Reds to go 8-6
Giants to go 9-5
Marlins to go 9-6
Ignoring the Phillies, who by most basic metrics are Good and have a very low bar to cross, missing the playoffs after finishing 6-9 would require two of the four teams above, all with worse records than us, all with a negative run differential, to play .570 ball or better the rest of the year. Certainly possible in this very pessimistic situation. Change the Cubs record to 7-8 and suddenly you're looking at 9-5 x2, 10-4, 10-5. Basically: close to .500 and we're fine.