If Spain win against both Chile and Liechtenstein they will return to the top.
If Netherlands win against both San Marino and Finland and Spain tie Chile and defeat Liechtenstein, both teams will be tied for 1st in the FIFA rankings.
Assuming Spain win against Liechtenstein and Netherlands against San Marino, then this is how the totals can change:
Netherlands (L: 1480, D: 1510, W: 1571)
Spain (L: 1554, D: 1571, W: 1605)
If Spain lose against Chile, Netherlands need to win against Finland to remain alone at the top.
I ran 10000 simulations for the matches taking place between now and October 12th.
Netherlands alone at the top - 10.27%
Tied with Spain - 17.11%
In second place behind Spain - 72.62%
Netherlands alone at the top - 17.98%
Tied with Spain in 0.07%
In second place behind Spain - 81.95%
Netherlands 1st (or tied for 1st) in both September and October - 8.1%