Do AFCI/GFCI receptacles protect downstream circuits without equipment bonding conductor?
Yes, GFCI decices don't require a ground to function, they don't even use ground. 

If current is following a third path, causing a trip, then who knows what the third path is?  We just know it shouldn't be there.  The assumption is that an unwitting human is the third path.  Maybe they're touching the equipment while leaning on a water pipe.  

AFCI  is a totally different thing than GFCI.  GFCI protects humans from shock, AFCI protects the building from fires due to arcing wires in the walls. It does that by listening for the "hooking up your speakers with the power on" sound.  AFCI works best in the panel; it defeats the purpose to put it past the wires it's there to protect.