Every single vent hissing/leaking at low pressure

I'm not at all sure about entering into this thread. I do not think you have a vent problem. Hissing vents are usually a pressure problem, so I suggest you start with a low pressure gauge on your boiler and verify that the pressuretrol is, in fact, opening at not more than 2 psig.

A panting vent is almost always indicative of a pool of condensate somewhere in the piping between the boiler and the vent in question. This would also keep the radiator from heating, or at best make it very slow to heat. Check the entire length of the piping from the boiler to that radiator and ensure that not only is the pitch adequate from end to end but that there are no sags.

If you can do that without attacking the manufacturer and the supply house and reply back, maybe we can continue.

I believe @offdutytech is in the metro detroit area.

I was just going to say it could be wet steam. What Steam Doctor just said. They do have trouble staying closed with wet steam.

Put a 6 in. engineering scale on your sight glass with some electrical tape. Make a note of exactly where it is at before the boiler fires up. Watch what happens to it during a whole tstat call for heat cycle.

When mine is working properly it only goes down maybe 1/8 to 1/4 in. from the initial level in steady state operation. Anything more than that and I would strongly suspect wet steam, I mean excessively wet. I saw that yours looked pretty low but I don't think your vids made note of what the initial level was. 1/8 in. in my boiler is one pint of water and multiply that by 1,700 and you get 57 sq. ft. of steam.

You may have areas where your pipes are sagging and those will just be pools of condensate making your steam that hits your radiators very wet. The only way to check is with a short magnetic level and bite the bullet and slit and temporarily remove all the insulation in the supply mains and runouts to the radiators.

I don't know if you ever checked the height of the top of your close nipple on your Hartford Loop either. Bite the bullet and run a string around the boiler with a level and some tape and see where it comes in relative to the NWL. Keep in mind that if the water level goes down in operation you are exposing that Hartford Loop and you really don't want it to be doing that. Depends upon what height was put in at by the installer.

It is good that you don't have an auto feeder IMHO. Those things can destroy houses when they fail. The manual feed should be repiped some time in the future to come from the hot water side of the water heater, with a backflow preventer.

You have done an excellent job relative to most posters laying out and presenting your situation and I can certainly understand your frustration after buying all those vents to see them all leaking. Vents "should" be able to handle a couple psi without failing, but, often they don't and seemingly more so with recent Hoffmans and Gortons radiator vents which were good names at one time. We have seen some failures on Gorton main vents as well but less so and usually as received and it does seem yours were working at least initially in your first video. You can try to see if Supplyhouse would replace the vents (suggest with MoM's for the radiators) but whatever you do put on there is going to need a little TLC in the system control. Early on you showed your sqft radiation vs the boiler rating and it is pertinent that you are indeed 30% oversized. The absolute sqft may seem small but it is the percentage that matters. I too have a similarly oversized boiler and the one thing I have learned is that we have to be more creative in keeping pressure down and what I mean by pressure down is nowhere near the pressuretrol cutout and really shouldn't ever be more than what you see once the boiler starts producing steam and the main vents close. Practically speaking that is less than 0.1-0.2psi. This translates into trying to keep your thermostat demands in line with not needing to fill every radiator with steam to the point that it's vent needs to close on steam. This is not easy but there are ways to succeed. Minimizing swing temperature if your thermostat has that as a setting, increasing the cycles per hour and initially eliminating setbacks. You can start experimenting with setbacks once you have reached a balance with steady state constant temperature control but you will find they need to be broken up into small increments or you will be banging off the pressuretrol cutout opening yourself up to potential problems with vents. Certainly the returns should not join above the waterline but that is more toward establishing balance than protecting your vents.

So, see what you can do about politely trying to get some compensation from Supplyhouse via new vents (no guarantees but it has been done), keep your pressure way down and next summer look at repiping those returns. Everyone here does want to help and are doing so freely on their free time and it really is quite a knowlege base. There are dozens of cases going on at the same time so a few pics or videos or comments will get missed from time to time. Have patience and play nice.

Wet steam definitely has less enthalpy and it can't heat the bimetallic spring in the Gorton vents as well as dry steam, even though they are both at saturation temperature. The moisture entrained only has sensible heat and can't release the latent heat of vaporization that the steam can, to heat the bimetal.

Even though I didn't see a lot of surging in your sight glass (mine gets much worse when dirty!) I wouldn't be one bit surprised if your water is dirty and/or oily. I think a drain and reverse flush of the boiler and your wet returns is in order and a refill. I'd use distilled water and Rectorseal 8-Way at maybe a light dose to start to see how your system handles it first, and to make sure you avoid alkalinity foaming. You could experiment with higher doses up to or even slightly more than the directions in the future, with pH testing.

I recommend the distilled in order to eliminate hardness as part of the problem and chlorides are bad for boilers anyways.

that doesn't mean the tapping in the boiler is clear or that the gauge is accurate