I don't get why you should split the cost of repairs? The home had these known issues, they didn't disclose it, and now you offer to pay 50% of it??? Again you bid on a house that didn't include these issues, correct? If you would have known up front about these, I guarantee your offer would have been different.
The way gas pipes are, if there's a puncture mid run, you can't just fix that one single pin hole without disassembling the entire rest of the run. Unless they use unions which aren't up to code. Whatever though. Glad it was fixed. Just the idea of the sellers fixing things doesn't sit right with me. It's going to be your house, hopefully for many, many generations. You should do the work and know that it's done right. The sellers have already, and will continue, to put bandaid fixes. For example, this gas issue was clearly behind drywall. Did they repair the drywall and texture and repaint? I highly doubt it. I mean again, it's safe and whatever, but it's all stuff you should say "How much is it going to cost? Okay, give me the credit and I'll do it. Let's move forward" I bet you will have to re-do everything they touched at an even high cost after moving in.
I have a feeling the sellers are getting really tired of all of this by now. You are also putting in a lot of time, money, and effort. AND YET there hasn't been one hint of a price reduction mentioned by anyone. The sellers are prolly thinking, "I fixed everything this buyer wants, let's close now" and here you come with "I want $XXX reduction" and they may be blind sided and really mess up the transaction. When I buy homes, any issues that came up, I have a blunt discussion of cost and move forward for me to fix later. It's both faster to close and less stressful than this.
Now keep in mind, I am doing a shit load of monday morning quarterbacking. I am also typing from the comfort of my home without repercussions. I most likely don't have a clue on the real temperature of the deal. So as always, take everything I say with an elephant sized grain of salt and trust yourself.
Best of luck Jerome!