Black iron union is the answer- to me its difficult to explain unless you're looking at the situation. Komocabo, your advice is fine, evidently it worked for you. Teflon gas tape/dope is necessary.
Folks please be very meticulous if you are DIY'ing gas pipe. If at all possible...
The sulfur will dissipate with time. A diacetyl rest just means warming it up before you lager it. Maybe 62 F or so for a few days. It will help eliminate any diacetyl that may have formed earlier. What yeast did you use?
Probably want to do a diacetyl rest in case it put out any off flavored at that temp. I've had good luck letting mine cool for up to 24 hours before pitching.
I'd sure try to redo those before I'd take it apart. If you wipe the joints and reapply solder when you reheat the joint it will draw the solder into the joint. As the existing solder begins to melt apply more solder to the joint. It won't be pretty but nothing short of starting over with new...
If you are heating joints that have already been soldered you will want to apply more flux to the joint (without taking the joint apart) after you've heated and turned it. Then reheat and flow more solder into the joint. It is going to be hard to not scorch the joint and it will not look...
I would up that sulfate level a lot, like to about 250 ppm. A little more calcium would not hurt either. I like Martin B's profile for pale ale on Bruunwater. I am also jealous of your existing water. You could build almost any water w/out distiller or ro water.
You will seal the weld less bulkhead with the o ring. Tape won't help. As far as testing black iron connections ideally you would use compressed air and a pressure gauge. You do not want a slow leak filling the room with gas.
I bought a ten gallon megapot for my bk. Really wish I had gotten a fifteen (and I only do 5 gallon batches). My hlt is 30 qts and my mlt is a 48 qt cooler. I batch sparge and they are big enough.