Losing a ton of water in the boiler

Yes. The whole idea of the exercise is to vent the mains -- and the dry returns, although a vent location on each main anywhere beyond the takeoffs to the radiators will do -- as rapidly as reasonably possible. Then all the radiator vents have to do is to allow air out of the radiator and steam in, and you can balance the system and get the heat output on each radiator just where you want it with the individual vents.

I would suggest you reevaluate the way you are looking at the venting chart. Look at the lowest pressure listed and work from that. The reason being is that to get pressure you have to slow or stop the steam flow. Yes at 3oz the vent flows more, but you are slowing the steam down to do that. The boiler doesn't make pressure, system restrictions relative to steam production makes pressure.

I would for sure go with the #2. The #1 is 1/3 the price and roughly 1/3 the capacity.

What @KC_Jones said. Vents close when hit by steam. That's their purpose. If they don't close, they let the steam out, in essence, releasing water out of the system. Your question as to what happens if tstat was not satisfied when vents close is why everyone here advises against overnight setbacks in excess of 1-2 degrees. Steam systems can takes a while to swing temps up, which can cycle the boiler on and off multiple times, which is not as energy-efficient as a constant on cycle.

Steam is invisible, you won't see it. You could be dumping steam and not know it. Put a cold piece of metal in front of vent and if it comes back wet that means steam is coming out.

@midiman143 , What brand vents are you using on those radiators? Did you buy them from the Home Depot or Lowes? If so, they are likely cheap Chinese products and they simply don't work well.
You have small main vents on your system. You can't balance your radiators until you first get good main vents on the system
When your radiator vents continue to hiss, are the radiators hot all the way across? If they are not, then the vents will likely remain open and that is fine. It is usually only on the coldest days that a radiator will get completely hot all the way across and close the vent.
Do you have a 0-3 PSI low pressure gauge on your boiler? You really need to see what pressure your system is running and the old 0-30 PSI gauge just won't let you see what is happening down in the range a steam boiler should operate at. Install one.
Once you install that low pressure gauge, it is completely possible the Pressuretrol needs to be calibrated. Most of them do. We can provide you that procedure, when you are ready for it but it takes a low pressure gauge to properly set it.
As for where the steam/heat goes when all vents are closed, the steam condenses in the system, mostly in the radiators. That process is what creates the heat you feel. Once it condenses and goives up its energy(heat), there is now room for more steam to enter the radiator and repeat the condensation cycle. A properly sized boiler will produce about the same amount of steam as your connected radiators can condense so the would never build enough to cause the Pressuretrol to shut the boiler down. The Pressuretrol is then just a safety device to ensure the boiler pressure never gets out of control. The boiler would only shut down because the Thermostat is satisfied. Unfortunately most boilers are somewhat oversized, meaning they will produce more steam than the connected piping and radiators can condense. That's when you build pressure (more steam in the system than it can process). Then the Pressuretrol becomes a working part of the heating process and it shuts the boiler down when the pressure builds beyond the desired setting, typically about 1.5 PSI. When enough condensation occurs the pressure drops to the cut-in setting and the boiler re-fires.
Adding enough Main vents allows the steam to push all the air out of the mains without back pressure. That gives you a few minutes of extra operating time for steam to get to the radiators before pressure starts to build (on an over-sized boiler).

@midiman143 , the Hoffmans and Watts are good brands of radiator vents but something is wrong if they don't close, especially if none of them close. The Pressure seems good. Are you 100% sure they are not closing? You would see lots of condensation, almost like a spray coming out of them if they are open. I'm thinking what you hear may well be at the end of a heating cycle there may be enough of a vacuum in the system that it pulls the vents open and what you hear is actually air being sucked back into the system. Your new Main vents should help that.

@MilanD , I did just look at that video again and I have to say I don't believe steam has gotten to that vent yet. It is hissing as if the pressure is well above what the OP says his gauge indicates but, when a Hoffman 1A fails to close, the spray from condensation is like a mist fountain that sprays about a ft in the air and about a ft. diameter. You don't need to hold a mirror up to it, especially at the velocity that video implies. I'm baffled.

@midiman143, in reality there should be no gurgle. That is an indication that there is water near the vent or maybe the vent is water logged. In that case, make sure the radiator is pitched back towards the supply valve.
The hissing really shouldn't be there either. You might hear a mild flow of air but the kind of hissing I hear on that video suggests that the system pressure is well over what it should be or that you have the vents open to the max, which should hardly ever be needed, except maybe on a huge radiator where you want a lot of heat. On those Hoffman 1A, setting 3 or 4 should work fine.
There is no defined amount of time for a radiator vent to close. By the time the last section gets mildly warm the vent should be closed.
AGAIN, THE HISSING PROBLEM WILL LIKELY GO AWAY WHEN YOU HAVE YOUR MAINS PROPERLY VENTED. WHAT IS HAPPENING NOW IS THAT THE RADIATOR VENTS ARE TRYING TO PUSH ALL THE AIR OUT OF THE MAINS, THE RADIATOR RUN-OUTS AND THE RADIATORS. THOSE MAIN VENTS YOU HAVE ON THERE NOW JUST DON'T HAVE THE CAPACITY TO DO THE MAIN VENTING AND THE RADIATOR VENTS WEREN'T DESIGNED TO HANDLE ALL THAT AIR.

@MilanD I have Hoffman 1A's on all 15 of my rads and not one of them ever gurgles. A few did at one time and it was always remedied by re-pitching the rad. They hissed once upon a time when the pressure was out of control and when I only had one Hoffman 75 on each main.

It must be something in the way the system is laid out. My 2" steam main circles the chimney so it's only about 12 ft long. As a result the runouts to radiator risers are 12 to 15 ft long.

About 7 years ago I bought a 6 pack of Hoffman 1a's and put them on all the radiators but the bathroom which had an ond Ventrite on it. All the hoffmans spit and would only seal if I rapped them with something. I went over all the pipes and checked the pipes for slope, the boiler is set to cut out at 12 oz and back on at 4 oz. The boiler is oversized so it does cycle some before the thermostat trips.

I tried a Maid O Mist on the bedroom rad and that solved the problem, I then replaced all the Hoffman's with MOM's and everything is happy. So now I have a bunch of hoffman's sitting in a box along with one spare MOM.

Bob

That's great on stopping water loss - I am also surprised how much water a boiler can lose from the main vent or the rad vent. We have some 30 radiators and 3 mains, a total of 5 main vents (3 big mouths and 2 Hoffman 75). I walk the premises every few days to see if any of the vents is hissing/leaking. They are not, but, on these cold days when boiler is on a lot, I easily go through about a gallon every 2-4 days... I've discovered since my last post here, that my Big Mouths tend to puff a bit before they close, but I'm for now will not ask for a replacement disk... they do close eventually. At any rate ...

To your pressuretrol calibration: All you need is a good 3 psi gauge on your boiler controls arm, and then watch the boiler run when the pressure is building. Once it gets to the pressure you want cutout to happen and it ptrol does not cut-off, you can use the set screw inside the ptrol's pressure push arm at the bottom of the inside of the trol, to calibrate cut-out pressure... There is a blue lock-tite you'll have to scrape off, and you'll need a small allen wrench - i believe it's 1/64 or 1/32... it's really a small one. You do righty-thighty to screw it in and this should lower the cut-out pressure. Be careful not to push the screw all the way through as it can fall out.

If you want to run even lower pressures, getting a 16 oz vaporstat might be the way to go, but how well that works will depend if the boiler is properly sized and if you in fact can get rads hot properly for the given outside temperature without building the pressure first. You have an oil boiler, right? I have no experience with oil so can't comment on benefits of running on slightly higher pressure vs. cycling on lower pressure on oil fuel.