Ok, a few things:
First, SAS support is better on DS3617xs than DS918+. You should consider moving to 1.03b and DS3617xs for the best results.
It looks like you decided to uncomment the grub command set sata_args='SataPortMap=4'
If you stay on DS918+, I would try the following: set sata_args='SataPortMap=1 DiskIdxMap=1000'
The SataPortMap argument tells DSM to only use 1 slot (the loader) from the first controller, then DiskIdxMap assigns the first controller to slot 17 (effectively hiding the loader), and the second controller (hopefully your passthrough) to slots starting at 00 (/dev/sda).
If for some reason you decide to switch to DS3617xs, the DiskIdxMap parameter should be 0C00 instead of 1000.
SAS controllers sometimes behave weird with this setup, but let's try it first and see what you get.