I have been a victim of false allegation. Nothing that has ever happened to me hurts me more, and I came out of it relatively unscathed from a legal standpoint. My daughter bragged about her lie on myspace and described why she lied. I hacked into her account and saved the transcripts, and she confessed anyway. So the case was dropped.
The thing that is going to hurt is the 5 year delay. Many people will draw conclusions that your husband should have been more diligent about remaining in his children's lives. Gecko made that comment, but she will not be nearly as derisive about it as his ex's lawyer. This is an ugly war, and you'll need thick skin.
What were the reasons for waiting so long? If she moved so often without notification that could be a valid defense, unless years had gone by before she moved without any effort to see the kids.
Why were you unable to get a court date?
Sometimes there is more to the story than you can wrap up in a single post, but please continue to post. We are listening and we are here to help. There are ways to overcome the alienation of your stepchildren's mother, but it will not be easy.