Yeah, specs say 4 SAS ports only through the external connector and 4 internal SATA ports. The external SAS port can be used for SAS or SATA, the internal SATA ports are SATA only.
The external connector assumes you're using external enclosures. If you want to you can loop it back inside the case but it'll require the case to have up to 8 SATA power connectors which is slightly unusual (assuming you're using the internal SATA connectors as well).
I'd have to read the documentation more for that board.
SAS drives can only connect to SAS controllers, SATA drives can connect to SATA or to SAS controllers but SAS drives cannot connect to SATA controllers.
If the 4 SATA ports goto a SAS controller then you could have 8 SAS drives connected at 6gbps or 4 connected "dual channel" at 12gbps or 8 SATA drives at 6gbps.
SAS is an extension on SATA. You can connect 4 SATA drives to most SAS ports or 2 12gbps SAS drives. You can't connect a SAS drive to a SATA port.
J12 needs a mini-SAS breakout cable and each drive requires a PSU. J5-8 are already SATA ports. 8 total SATA drives or 4 SAS + 4 SATA drives are supported by the ports given.
Some Dell servers require HDD/SSDs with Dell firmware locking you to their ecosystem. No idea about workarounds but I assume they exist when the factory warranty doesn't matter anymore.
Dell should tell you in the specs if you input the service tag to their service tag finder.
Where's the rest of the RAM slots? Should have 8 per CPU, not 8 total.
Where's the rest of the x16 slots? 2x Xeon's for that gen have 80 total lanes, I only see 2 x16 slots which you can use just fine with 1 CPU.
My 2687w's score the same R15/R20/R23 multi-core as my single 5900x. Kinda sad really but with 256gb RAM and either 4x 12gbps SAS drives or 8 6gbps SATA drives and up to 4 GPU's it's still a beast, abit a bit power hungry.
As for the AIO's...you want 2 dedicated headers for the pump that you can set to 100% then put the curves on the radiator mounted fans. As the other poster suggested I'd try to get both radiators on top rather than the current setup, that way you'll still have intake/exhaust fans to try to keep the rest of the system cool.
15min with a sharpie, a drill and tap says any 2U case fits as many ITX systems as you can physically fit in the space. Then probably 2-3 hours working out the wiring so the redundant psu's power them all.
Why bother with multiple motherboards when you can just have 1 and many VM's?