You can pass audio between the two desks using the Waves cards (or via Dante cards installed in each console if you needed to use Dante instead of Waves).

So your monitor desk will have the GX4816 (with the DX168s connected) connected to the built in SLink port. You’ll likely have a local computer running Waves, but you will also need to connect to the FOH Waves card as well. You will need to use the routing tabs on each console to properly route the needed I/O between the consoles.

That would mean I could use those Waves tie lines from Mons to FOH as record inputs too, then the remaining channels of the 64×64 stream can be used for plugin send/returns.

That is exactly what I was thinking. So if you max out the console at 48 channels, you would still have 16 sends and returns for plugin use. If you need less channels, you’ll have more sends and returns available.

Can I use 64×64 at 96k?

Yes, the SQ Waves card runs 64×64 at either 96k or 48k. There is no difference in I/O capacity between the two choices.

Does this mean I will need Waves approved switches at Mons and FOH to be able to have computers at each end on the same network to pick up those tie lines?

I think given the number of Waves connections you are going to need to make and the limited number of Waves ports available, you are definitely going to need to use some sort of network switch that is known to work well with Waves.