After more than a month banging out heads, we've reverted to 6.1.3.1 and near-perfect coverage has been restored.
After site-surveys and ARM setting jiggling, and client setting jiggling we're convinced (I and my Aruba Partner) that there's a bug in the 6.2.0.2 code which is causing the controller to mis-read/calculate/interpret the client at a low RSSI value and "assist" it off onto another AP. Turning off handoff-assist merely hid the problem.
Our warehouse management asked us to roll-back and wait for a more definitive fix/plan from Aruba and we went from 80 THOUSAND low RSSI deauths per day to 9 HUNDRED in 35 minutes (my record time for converting 6 controllers and 600 APs)
TAC telles me that 6.1.3.8 is getting ready for release and contains "a Low RSSI threshold issue."
Looking forward to the fix.