Purpose of exposed ground pads
It's likely these exterior (gold bare metal ) pads are to bond the layers together to form a nice faraday cage in situ (as mentioned in OP). Since there are some high speed signals your probes will require adequate grounding during test so they are left open for the probe card to be able to get a clean ground reference.  Some have vias in them because they bond through, some don't have vias because the ground is needed close to the pads that are being tested but there are traces underneath.  Particularly on the LHS.