I don't see why not. It holds liquid at greater than 5 psi fine.

The specs say it can hold up to 60psi when moving liquids.

I don't know but imho I would stick with fittings designed for liquid for liquid and those designed for gas for gas. It would be a bummer to have a tiny CO2 leak with this and waste a bunch.

I would say no only because of the even low positive pressure leaking past the orange. I have to change mine about every 10 uses for hot wort not under pressure.

Yes.

With some adaptations I use it to pressurize my conical to 15 psi.

As someone answered before these threads are tapered. Use plenty of teflon tape several layers. Stainless is very hard so the threads will not mesh easily like brass. Also NEVER tighten these up without the teflon tape as stainless on stainless has a tendency to gaul and basically weld the two pieces together.

The other answers are probably sufficient but to summarize NPT is a tapered thread. You need to use a lubricant/ sealant like Teflon tape or pipe dope to get the right fit. You need to tighten past "snug" (there are even torque values to reach if you want to look it up and be exact) You are actually deforming the threads a little to form a tight seal (which is why you need the lubrication of Teflon). The threads go well beyond what is needed to make a seal it is about halfway and there is an engagement distance you can look up as well.

I'm not an expert with the terminology but I have a lot of the SS quick connects throughout my system. I haven't had any issue with leaks. I'd try to tighten it beyond snug. I think the NPT (National Pipe Thread gradually increases in diameter to allow for a very tight fit).

I had this same issue when using SS ball valve to a 1/2 MPT to MPT. It only threads about halfway too. However when I use the Teflon tape and tightened it all the way down it does not leak.

pipe thread (NPT and MPT) has a taper to it so you have to use the Teflon tape and just snug it up