I hadn't read the original thread, but I've read all of this one with steadily increasing amazement, bewilderment and a dash of horror at how obstinate somebody can be in the face of overwhelming evidence that they are wrong. The one thing I will sort of side with the OP on is that her ex going after her for maintenance is a bit of a dick move...but even then, the OP spent months pursuing an obviously baseless claim against him for money he didn't owe, so she's not got much grounds for complaint if he decides to pursue her for money he is actually entitled to.
OP I appreciate that I may as well shout this into a bin for all the good it will do, but: if you decide to ignore the fact that - so far as the law is concerned - you do owe him maintenance, then the consequences are potentially a lot more serious than you ignoring the fact that your claim against him was baseless. The worst that could (and did) happen there was you being told 'no', and getting some stick on MN. The worst that could happen if you fail to pay child maintenance is, ultimately, prison. That's not likely, but please don't dig yourself any deeper into this horrible hole, and use the payment of whatever it is you are judged to owe as an olive branch.