Personally, I don't think you will have any problems in court. First of all, Ketron would have to deny you your warranty before it got to court, and this would be a PR disaster for them if they DID deny your claim whilst inside the official warranty period.
Secondly, Ketron have acknowledged the issues, and posted many times that a 'fix' is on the way with each OS upgrade they brought out. So they can't deny the problem, and it's easy to see they HAVEN'T fixed it yet. Their emails and posts here are proof that they are aware of the issues, and the fact that they keep promising to fix it with an update (but have failed miserably, so far) is all the evidence you need to answer why you have hung on to it for so long. A warranty period is not graduated. It is as much in effect on the last day of its' term as the first.
Get your money back. Start the claim now. You will probably STILL have time to load OS4 in and see if it DOES fix it before the RA comes in...