1) Doublecheck that I found the correct spec sheet.
2) Doublecheck that I identified the correct measurement.
3) Re-read all build installation documentation and specs very carefully. The fan can be moved up per Spec Bullet 3, but note the corresponding warning regarding chassis clearance.
You can also always move one fan to the other side and use a pull configuration or if you have a little space in the case move the fan up a bit on the cooler.
a. because it is a single fan version, ram with tall heat spreaders are ok.
(If you have not yet bought ram, do not pay extra for fancy heat spreaders, they are mostly marketing.)
b. The NH-D15s is 160mm high vs. 165mm for the NH-D15.
c. The NH-D15s is offset a bit to clear graphics card backplates when mounted in the first pcie x16 slot.
From a performance point of view the D15 is a bit better, but not enough to be meaningful.