I just learned that my friend and her mother have multiple lawsuits from debt collectors and original creditors. It made me thinking if it is possible that I can get sued as well. Here is my story:
I had an account with HSBC before. It has a balance of 800+ when I closed it and decided to pay for it in installment. When I was already ready to pay them, that was a week after my college graduation (2007), they have informed me that the account was sold to a CA.

I tried to work out with the CA, but have found that they have violated my rights. I wrote their legal department to conduct investigation. No reply was received.
I called the General Counsel and was informed that the letter was lost. He sent me a fax letter. In that letter was an apology and a settlement offer. Here is the copy:

I have then again called them and spoke to the General Counsel. I informed him that his letter did not solve any problem and instead introduce another violation. He said he "get what I was saying, and was sorry." He promised not to call again. I have never heard from this collection agency again. The collection agency also did not report any bad rating in my credit report.
I am ready to forget about this account but am worried, is there a way that I can be sued also by HSBC? What should I do to protect myself? Should I try to negotiate to the CA? HSBC would not want to talk to me. HSBC does not want to accept payment too, because according to them the account is no longer in their database.
What document should I keep? Help!