As an inspector, I understand that your inspector was probably trained to call for replacement of copper gas pipes, although it is unlikely to be a specific code violation. Copper is allowed under the National Fuel Gas Code and other similar building codes and has been used in many different states for many years. Years and years ago, there were more sulfer additives to natural gas which made it smell (helping you know there was a leak). This created copper sulfide which caused the pipes to flake and orcies to become blocked (among other problems).
In more recent times, the additives have been altered ro reduced and flaking is not nearly the problem it used to be. Still, most inspectors are taught to just call it out for replacement (since they are not master plumbers and don't often get down the real underlying resons why).
It is perfectly safe for Liquid Propane (LP) gas use.
Now, on to the bigger issue. This is a whole rengotitaion on both sides, and it helps to keep some of that in perspective. If you didn't tell them those were your most important items, then you can't assume they knew how highly you valued those repairs. It sounds like smaller, less expensive issues, and you also have to remember that the seller probably also came down in price in the first round of price negotiations, so from their perspective they are thinking "Just how much is enough for this buyer?" The process can tend to result in hurt feelings, but in the end, once you get down to the last few hundred dollars, it should all be able to be worked out.
Most buyers I have worked with actually prefer that the sellers do not make repairs but instead offer a credit so that they can do the work themselves, or have contractors they trust do the work. Let's face it, you just aren't going to take the same care with a home that you know you are leaving in 5 days as one you plan to spend the next several years in!
Finally, the soft copper can be replaced with CSST (corrugated stainless steel tubing) which is flexible and easy and cheap to install, for a reasonable amount.