It sounds like I have a very similar set up to you. Definitely do not need multiple 30psi regulators. I have mine turned down...not sure how much...but I suspect to around 15-20 psi. Catt22 has described the procedure I use exactly. I agree with him that the safety is likely whats causing...
I think you have just about everything you need. I used everything you plan to and my set up works great. The one thing I see missing is the pipe dope used for gas fittings. Have fun!
My old man uses copper tubing for the air lines in his shop. I'm not advocating it, but he's successfully fed 120psi air through the system for 8+ years without issue. So long as you are regulating the pressure out of C02 tank to something that you could serve beer at, you should be fine.
I just listened to a Brewing Network Podcast where this exact question was asked of a 5-Star chemical engineer. He didn't recommend it, although he said he'd done it, but using distilled water. Their recommendation was glass for long term.
The ball valves will work, but if your looking for easy tweaking, you may want to get the needle valves. I run the same burners with 1/2" pipe feeding them. I adjust with a 1/2" ball. It can be done, but it's a pain to get just right.
If I'm reading their catalog right, they are rated for 2 psi. They might work, if your using low pressure. I myself snagged up an 8210 model series. They seem to work well with the ~30 psi propane I have them controlling.
I had the same problem. I ended up getting a stainless steel flex gas line kit from Home Depot. The kit has 2 3/8" to 1/2" adapters, but I ended up only using one. Not a cheap route. I think it was $22/kit