If you choose to respond at all then you might want to tell your inquisitors that you have not given them permission to access your private mail so any 'evidence' they claim is possibly inadmissible. Also, you say they have given permission for you to use your computer for private use - be sure to have this in the form of a document should they try to deny it or have reliable witnesses, if possible.
You should take your lawyer to the hearing, he will be highly tuned to every nuance of what the employer says and it will be better for you, in the long run, if he ties them in knots and makes them look foolish.