Having been an avid sailing anarchy reader for the last 5 years I've finally found something worth commenting on!
I heard Nereus implode 10km away while I was reading in my rack. I had a hand in dissassembling her main electronics housing less than a week ago to replace a hard drive, I helped co-ordinate the predive checks, and I've spent the last 40 days on the R/V Thomas G. Thompson supporting her day and night. And I can honestly say from experience that losing Nereus was nothing like losing a friend. We will indeed have the ability and know how to build a new better prototype with time.
The oil compensated systems used carnation/mineral oil and the thursters/HPU used a special custom mixed super low viscocity marine hydraulic oil made by Royal Purple to handle these extreme depths. There was very little syntatic foam on the vehicle, the buoyancy came from 3.5" ceramic spheres. Since the failure modes of brittle materials can be very unpredictable, there is obviously a huge level of risk involved in sending a vehicle to 10,000m, and this is exactly why there are no other vehicles out there that can do it. At some point you've got to accept those risks rather than spend millions on destructive testing.
I've also taken apart Cameron's Deep Sea Challenger vehicle after he donated it to WHOI and that was basically just one huge and expensive movie prop. The only way he can possibly relate to our loss is that he also invested many many millions of dollars (3-4 times the Nereus design and build budget) on something that is now just as useless as Nereus.