Two Stage Propane Regulator with Splitter

I cannot fully answer your question, but it seems that New Hampshire simply uses the ICC guidelines for building codes, including for "Plumbing, Mechanical and Fuel Gas" which seems to be the key area here.

Not being an ICC member, I can't access the documents, and neither the New Hampshire Statutes website nor the State Building Codes website reprints these documents for public viewing. This may be to encourage the use of licensed individuals for work subject to code, but IMO as "ignorance of the law is no excuse" in most jurisdictions you should be able to educate yourself on the building code free of charge, the same as for any other section of law. I would call your county clerk and ask how to get ahold of a copy of the codes. It may cost you a little money for their time and paper, but if they can help you here it probably won't be as expensive as an ICC membership for yourself.

If the government won't help (excuse my cynicism but I'd put money on various offices passing the buck), if you need chapter and verse, just get it from both the propane guy and the gas company. They're telling you two very different things, and they can't both be right. Ask both companies for a hard copy of the section of the building codebook, with title and year of that document, detailing the relevant codes which back up their understanding of how it should be done. Both are working for you, and one has shafted you; my money for which one is with the one who can't quote the most recent official codebook recognized by the State of New Hampshire.

Corrugated pipe is generally intended only to be used as the last connection to an appliance, in a living space where it not vulnerable to banging or jostling (usually behind or in a space at the bottom of an appliance), but where it can be seen and accessed if work is being done. It is not intended to be buried in a wall, where it could be pierced by a random fastener driven blindly.

It is also only used where an appliance needs to be moved to connect or disconnect it to the supply line. In all other cases and places, solid black pipe should be used, not corrugated. Solid should be run to the connection right before the stove, a cutoff valve should be installed at that point before attaching a short length of corrugated between the stove and the valve.

Having said all this, I join in the various suggestions that you engage a professional plumber (pipefitter). I know how to do it, I have done it in the past, and I would NOT do it again. The risks are very high, and a simple oversight (common to many a DIYer) is disastrous. The pros avoid those oversights because of long practice.

Please stay safe.