Adjusting the gap has nothing to do with the dwell, all it does is control at what point in the firing cycle the firing can is isolated from the air supply. Shortening the gap between the o-rings makes the firing can start refilling slightly sooner in the bolt retraction phase, but too close and the can will start refilling too early and will vent a little bit before the main firing seal in the bolt stop is sealed again. Smaller gap even than that and it won't seal at all. One of the bolts I am working on can mess with the dwell mechanically and the tail o-ring spacing is directly related to it, but it requires additional components to do it. Remove the extra pieces, and it isolates the dump chamber like the other multiple tail o-ring bolts.
Sure, any Ion bolt will work in a mech Ion - but not every bolt will work properly. Anything that doesn't isolate the dump chamber from the air supply will continue to vent when the trigger is held down, which leads to inefficiency and depending on your pressure and a few other factors the possibility of hot shots when held down.
Yes - devolumizing helps with efficiency, sometimes significantly. Bring the can or breech section (if not a classic style) to the hardware store and find some plastic tubing (nylon, vinyl, whatever) that fits snugly inside the can. Buy a foot of the stuff (should only cost a few bucks), and insert various length sections until you find the balance that you like best. Basically, at higher pressures the flow rate through the system is increased - sometimes enough so that it actually takes a lower total amount of energy to propel the ball at the same speed.
An SMAV can sometimes run well over 200psi. According to a conversation I had with Clippard, they recommend not running over 200psi because the internal seals have a tendency to start rolling and eventually tear when run at pressures above that. As a result of that conversation, I have recommended not running over 200psi because I don't want to tell people go ahead and have them end up with an unreliable kit. Personally, I've had a few that start to leak just over 200, and several that I've had near 300 without leaking at all. Same with a MAC 33 - had some that start leaking just above the 200psi rating, and others that I have run well in excess of 300psi with no leaking.
The only reason you would ever need an LPR on an Ion is if you devolumize it so damn much that the 3-way (mech or solenoid) can't handle the pressure anymore. The balancing in the system is entirely ratio based and scales directly with pressure.
The more efficient bolts you see in an Ion are not more efficient because they lower the pressure - they are able to lower the pressure because they are more efficient. Devolumize those systems and jack the pressure up and many of them become even more efficient (and faster).