I does, but the I believe reserve is being applied to one person (the recipient) while the gas owner is using the planned gas. Again, only he can chime in to clarify for certain what he meant.
ALL downstream seconds will function all the way down to ambient, not just balanced ones and not just usually. And upstream seconds are getting vanishingly rare (I also honestly do not know their behavior below design IP). A first stage uses the IP to close the valve, so at tank pressures below IP setting the first stage remains open. So it isn't an assume the worst in that regard.
To clarify, I should have phrased it as you (general "you", as an OOG diver, not you specifically). As to knowing when to leave the bottom... that is gas management. I know my SAC/RMV (pick your teminology), I know my ascent rate, I know my depth. I plan the gas needed to surface (with safety stop if needed), add 500, and that is my depart the bottom minimum. That is not addressing turn points/pressures for navigating away from the boat and back, etc., but in an emergency the big gas supply in the sky will give us all we need for the surface swim to the boat if needed.
To the original point... if that 500 extra psi (13cf) reserve will suffice for the OOG diver on my alternate, why wouldn't handing the same OOG diver a 13cf pony get him to the surface? Or if I had a catastrophic gas failure (plugged dip tube for example), wouldn't that 13cf pony still be viable for me?
Respectfully,
James