Mamba writes:

Does old propane go "bad"? When I removed some of the old piping,
there seemed to be a nasty smelling gel layer inside the pipes. Is
this a congealed version of propane? Do lines need to be reamed out
before reusing?

The smelly goo is the mercaptan odorant, with some waxy hydrocarbons
that distill and/or polymerize out of the product. This is quite a
normal situation. It doesn't need to be cleaned out as long as it isn't
going to get into your regulator.

The regulator for the line is the last item on the feed pipe before it
comes through the cabin wall. Do these fail from lack of use?

It's possible but at only 3 years old it will likely work fine.

If I replumb the gas lines, can a standard propane supplier inspect
the system or is there an offically body I need to get to inspect the
new setup? I don't want to mess with gas (or my insurance) without
having someone knowlegable who can verify my work.

A propane dealer will be technically competent to advise and inspect.
Whether it is up to some code will depend on the local jurisdiction.
Most "remote cabins" are not going to be inspected, whether it's legally
required or not. Between a sound installation and inexpensive
electronic leak sniffers in the living space, you should be fine.