I have an Avantis and tried using the GX4816 using the dedicated S link port, and things work as they are supposed to. I added the AR2412 and the AB168 with the S link card and the board would recognize them but things would appear with no phantom options and other problems.
So, would the correct solution be to have the GX off the dedicated SLink and then have a DX168 off the SLink card, and the other DX168 linked from the GX expander port?
Perhaps as an additional point:
The Avantis configuration you outlined in the headline could work if you connect the AR2412 and AB168 to the internal SLink port and install the available gigaACE card for the GX6416.