I think you misunderstood what he is saying, maybe..
The original description was confusing to say the least, and was a bit off topic since it started off talking about de-carbing. However, I think I correctly interpreted the part about the additional regulator itself being hooked up in reverse, which won't work as described to turn it into a spunding valve.
In the post, there were two end goals, carbing and de-carbing, and (at least?) three methods describing how to do them. One was scrubbing excess CO2 using more CO2 to agitate the beer causing a release of dissolved CO2. The equivalent of shaking a soda bottle example, and releasing the gas, only gently and continuously. Another described someone else's fancy technique of using a spunding valve to automatically release the excess pressure while agitating. Contrary to what was stated, the beer can be decarbed below the pressure in the head space, just like what happens to a soda bottle when you shake it. Yet another was a force carbing technique using the same basic gas flow as scrubbing, only to add CO2 into solution. This is like your method of putting gas in through the out door. This was then followed by de-carbing, presumably because he overcarbed(?).
To add some additional variations- You can carb even faster by combining the two, and continuously bubbling CO2 through the beer by using a spunding valve (or cracking the relief valve frequently) to allow a release of excess pressure. This creates a continous flow of CO2 to increase surface transfer. It wastes CO2, but would carb fast. The same overcarbing danger exists if the input pressure is higher than your desired carb level.
A spunding valve can also be used to reduce the carb level slowly, like a set and forget; or rapidly by agitating with bubbled gas, (kind of) like the high pressure shake, rattle, and roll method. The same caveats apply- going with either quick and dirty method you can over-decarbonate just like you can overcarbonate.
You send gas in through the beer dip tube to carbonate. The co2 bubbles up through the beer and dissolves a bit faster than it would just sitting in the headspace. I've done this and think it does help.
Yes, but only a tiny, tiny bit. Even with a stone, something like 90% of the CO2 is dissolved where the plume breaks the surface, unless your release rate is so low the plume doesn't break the surface, but that would make for an extremely slow carb'ing method. With the size of the bubbles coming from the dip tube, the plume effect is insignificant. In this case, the surface of the beer in the head space is where virtually all of the dissolving is taking place. With the set and forget method, one trick is to lay the keg on it side to increase the surface area.
But it is risky because the beer can backflow up the gas line and into the regulator. Of course this could also happen with the gas hooked to the gas post if you are rolling the keg (a typical method of quick carbing)
Be careful to never reduce the pressure from the regulator while the gas is hooked to the beer side. A check valve on the gas line is also a very wise safety precaution.
Yes, check valves are your friends. I don't know why they are so expensive ($8) for what they are.