They are legitimate questions however one can normally track the person by his IP address. Tracing an IP address is actually pretty straightforward, and even though it's not always possible to track down a specific individual, you can get enough information to take action and file a complaint. Those websites the asker mentioned here are well protected applications that one can not easily hack into them to tinker someone's information. Also the websites do register computer details while signing in & out and hence refresh its cookies accordingly, especially by smart Google search engine!
It seems, this person had created those accounts long time ago as per his story, therefore he would have received the confirmation mails to his email address too.
Suppose one never signed up with those websites then any inquisitive person of such would immediately react to the mails to see who opened these accounts on his behalf and take appropriate action against that culprit. If one denies that he never had such emailID, he can still be caught without any great effort by seeing whom he mailed or chatted with etc. Isn't it?
I am just contemplating on what he said as ".. innocent mistake..". But, one of his responses says "..I refuse to accept her.." Therefore, how can it be an innocent mistake? Though it may not seem that big of a deal, he may want to be aware of it while talking to the Judge. If I am missing anything here, please correct me.