Advice for collections nightmare

Hi all, could use some advice on how to proceed after having an apartment complex placing my account in collections, which was disputed and eventually settled but now still on my credit reports.

Background:
I was offered a promotion and subsequently would be leaving the state. My apartment lease made no provision for early out for job transfer, so the only option was to break the lease early . I met with the manager and asked him to tell me what I needed to do to walk away, as my lease was up three months after I would be vacating. I told him my company would pay up to two month's rent as part of the relocation package, but he said that since I had a prime unit (first floor, bonus room, next to the pool, etc.) they would just rent it out. Since I was moving in the first week of the next month, I would pay the full month's rent and then they would send me a check for the difference, once they leased it. They would rather do it this way since if he took my company's money for the remaining two months rent, they wouldn't be able to rent the unit out since it is illegal to charge double rent on a unit, and would create an administrative headache to work around. Win win all around, right? So I thought...

I never hear from them about the deposits or refund of a partial month's rent, and figured this is their way of retaining my deposits (common practice among apartments). Fast forward six months later and I start receiving a barrage of calls & letters from a collections agency, (as does my family, who were listed as the emergency contacts on the lease) stating that the complex has placed my account in collections. Not only did they charge me the full amount remaining on the lease, but also to recarpet the whole apartment, claiming it was stained beyond repair for an additional $2k in charges (which it was not). I disputed the claim with the collection's agency, and began contacting the complex via certified mail as I wanted a paper trail.

The complex's staff responded back and said that it was my responsibility to meet the remaining obligations of the lease (they had rented it out but not until almost a full month later, so they did reduce the balance to reflect this) and insisted the carpet had needed total replacement. I responded back appealing the agreement I had made with the manager, and asked for proof of the damage (knowing that there wasn't any) and copies of the billing statements.

They were firm on their stance, saying that they had taken photos of the damage done to the carpet, but they had all been destroyed by a virus on their computer. They did provide the bill , which showed carpet installation to a different apartment (which noticeably had been crossed out out with my apartment number handwritten in). They also included a statement from a carpet cleaning mom & pop shop which said that the carpet was damaged beyond cleaning and would need to be professionally replaced. However, this statement was dated a full month after the carpet was supposedly replaced by company A, and even after the new tenant supposedly took ownership of the apartment. Dishonest to say the least, if not outright fraudulent, in my opinion.

I made copies of the statements and escalated the matter to the management's corporate offices, explaining the background, and that I had never received a bill nor were any of the tenant resolution procedures followed for a late/past due rent (in accordance with state law), that they had simply turned my account over to collections. Furthermore, the proof they offered that I owed for these supposed carpet charges was suspect. Corporate responded that they would waive the carpet charges and would remedy my account with the collections agency if I would pay the partial month's rent balance. Considering it a hard lesson learned in not trusting what someone says if it isn't in writing, I agreed and paid the balance directly to the complex, including a copy of the letter from corporate.

This collections account now appears on my credit report as paid in full. I thought they would have removed it altogether, but even still I had no idea a paid in full collections account would have such a negative impact on my credit score (went from ~790-800 to 670-I've never been so much as late on a payment). I really have wanted to avoid hiring a lawyer as I don't want to incur more expenses over this, but I'm not sure what else to do. I'm not really after damages anyway, I just want this bogus account off my credit report.

I've tried filing disputes with the three major credit bureaus only to find out in a case of individual's word vs a business, they side with the business. I am sending another letter to the corporate office to ask why my account wasn't removed, as I still believe it never should have gone as far as a collections agency, but I'm not sure I have any more leverage other than legal action. Any advice or tips are greatly appreciated-thanks.