Problem with disputing credit report information

Sorry for the long post. I'll provide some cliffs at the end, but I feel like the details are necessary for context.

About 2 and a half years ago I was prescribed some medical equipment that I had to pay for out of pocket to meet my deductible. It was roughly $850.

I checked my credit card statement about a week later and noticed that I was billed by this company for two separate amounts. One was for an amount that was a few dollars more than what was specified in the equipment purchase agreement, and another was for $237. This company has billing departments in two different states (PA and KS), and one charge was from PA and the other from KS.

I immediately contacted the local office that sold me the equipment, and they were just as confused. They told me I'd have to call the 1-800 number and speak to billing. So I called, and of course the billing department is just a call center in India, and the only thing they can tell me is that this is what my account says I'm responsible for.

I ask for an itemized list of the products I purchased. They mailed it to me. The "itemized" portion of it is a joke. It all says "Medical supply" or "equipment" of various amounts. And worse, the amounts all added up to about $720. Over $100 less than the amount I initially authorized.

Instead of calling India again, I dispute the $237 charge with my credit card company. The dispute ends in my favor because the medical supply company either never responded or couldn't find any proof that the charge was justified or authorized.

To make matters even more confusing, I checked my health insurance account online to see what the company actually billed to my provider, and come to find out I was only really responsible for $616. I decide to let things be since the company clearly has some crippling organization and communication problems.

But then...A few months later, I get a bill from this company, claiming I still owe them $720 (the arbitrary amount that was part of the "itemized" list they sent). I call India again. They have nothing of value to tell me but they tell me someone will call me back. A week later, someone does call back. But it's an Indian from their collections department still trying to get that $720.

So, round and round that goes for a month, unresolved. Eventually, my account is sold to a collections company. Long story short, I try to explain to them the issue, I give them a copy of the original purchase order and a redacted excerpt of my credit card statement, and the collections company tells me they are marking my account as paid. Yay.

Then a year passes. August 2012, a new collections company calls trying to collect the same amount. I tell them the same story, send them the same documentation. I don't hear from them. Two months later, my credit monitoring service tells me a collections account has been added to my credit report.

I check my credit reports, and the account is on Experian and Equifax. There is a note in the Equifax report that says "customer disputes debt." Also, oddly, the debt for Equifax is listed as $580 instead of $720. The Experian one says $720 but doesn't mention any dispute. I don't think it matters either way. I feel like it's going to kill my decent credit score (which, last time I checked 2 years ago, was 760).

2 days after the credit monitoring notice, I get a notice from Citibank that they recently checked my credit and have decided to reduce my credit card limit to $500. (the limit reduction doesn't matter to me since it's an old account that I never use, but the fact that I'm already seeing effects/consequences is alarming, to say the least).

I go through the online dispute process through Experian and Equifax. I could tell by the "process" that this was going to fail. They basically make you select a dispute reason from a pull-down box and give you a whopping 150 characters to provide "additional information." I assume that it's going to be a very autonomous process where they ask the collections company to provide "proof." Surprise, surprise, the collections company has my account on record.

Equifax has already emailed me 2 days later to say that the dispute has been closed (I lost, of course). I'm Experian will be next.

CLIFFS
- Bought medical equipment two years ago. I was overbilled. Tons of miscommunication between their multiple billing departments.

- Company then claimed I owed another $720 but could not explain where these new charges came from, only that they were owed.

- Account goes to collections. I send collections agency a copy of my original purchase order and my credit card statement to prove it was paid. They agree to mark my account as paid.

- A year passes, and new collections company contacts me to collect this "debt." I again send the same documentation.

- Two months ago, credit monitoring service alerts me that a collections account has been added. I check my credit reports, and the account is on Experian and Equifax.

- I start the dispute process online. Very limited opportunity to provide information, and my dispute gets denied in a very automated fashion by Equifax, as apparently their "investigation" merely involves asking the collections company, who reported this account to them in the first place, to verify that they do have an account for me on file . I suspect Experian to do the same.
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So, what are my options here? Can I take this any higher? FTC?

Any idea how badly this will affect my credit score, which was 760 last time I checked? (basically, is it worth fighting)