This is what I do. Especially for an IPA that's a good idea if you want it to last more than a few days in the fridge.

Hmmm. Never considered using a cartridge, but pretty sure it should be enough for a 2L bottle. Only way to find out is try I guess... :)

I don't have the reference but I recall calculating somewhere that 16g should carb one gallon.

Ball Lock Gas (white)

Gas and liquid ball lock disconnects are slightly different but the carbonation caps are made to a tolerance that will work with both. So with the cap and tube fitted to the pet bottle you can flush it first with CO2 to remove the air !! Then then using counter pressure fill the bottle through the carbonation cap. You can use that method to a bottle with already carbonated beer. Or if you're in a hurry to try the beer before it's carbonated you can transfer the beer into the bottle in the same way, and then force carbonate it as shown in the video.

If there is no check valve near the cap (and there probably won't be one unless you put one there), then you may get beer running back down your line and even into your regulator, which you do NOT want to happen. So don't turn the bottle upside down! No problemo, it just takes a little longer to carb.

You can also remove the gas line if you're in a hurry and want to shake and turn the bottle upside down. Then re-attach the CO2 with the bottle upright. Rinse, repeat a few times.