i would not trust any company that claims that they can do this do that, unless the other company agrees to it. e.g. if ABC claims that DEF's contract is still valid, and that you are liable for maintenance, ask DEF directly or let CASE have a look at the contract...
similarly if ABC claims that they can terminate DEF's timeshare, pay only when you see a letter from DEF saying that the timeshare is terminated.
its as simple as that. a 3rd party [not even CASE or lawyers] cannot help you to terminate [or change terms & conditions] unless some laws were broken when you signed the timeshare...