Ex-employer trying to stop me from opening a competing company by saying I was involved in a conspiracy to steal various confidential information

If your ex-employer sues you, you may need to hand over at least some of that information during the lawsuit--it is the information he would need (and have a right to request) to prove you did steal, and which, conversely, you might rely on to prove that you did not.

Until there is actually a lawsuit filed in court, however, you do not need to hand anything over. And even after a lawsuit has been initiated, your lawyer will know what is proper and germane for the other side to ask for, and what your attorney should object to.

In terms of defending yourself, if your ex-employer  sues you, HE has to prove you did steal--not the other way around. Technically you don't even need to amount a defense or provide evidence supporting yourself, if you feel his case is so weak--though it's usually a good idea to put on a vigorous defense.

As for your ex-employer's tax evasion--report it to the tax authorities if you feel fairly sure of your facts and believe it your civic duty. It will not impact his attempt to shut you down or your defense of yourself--it's a separate issue.