I have not read this thread, however. I have a US Coast Guard submersible pilots' license (Only for a sub that goes to 300 feet deep). I say this because the submersibles I piloted, which carry 48 passengers and 3 crew had an emergency drop weight. This weight weighs 2-tons and when released made the sub buoyant and it would float to the surface.
This weight was mechanically released by the pilot or co-pilot from the cockpit and did not rely on electronics. It was a manually activated lever that if cycled a few times pushed a pin holding the weight and the weight dropped.
This weight was drop tested annually at the dock and was an requirement by the American Bureau of shipping and the USCG before she was certified to carry passengers. I am quite certain this Oceangate submersible is required to have the same thing.
The submersibles I piloted also had the capability for voice coms with the surface and we routinely talked back and forth to the surface support craft.