My wife is self employed as an LMHC and conceals 8-% of her cash proceed from the IRS and has been for 20 years. We are in the middle of a divorce
And I want to threaten her with the IRS to bring her to the table and allow the divorce to go through un contested. Can I do that in mediation?

Nope. You can not threaten someone. If you do then you could face charges. Also if you have been married to her and were filing a joint return then any exposure could lead you to owe the IRS also. You might be able to bring it out in interogacies and then have it as part of some calculation like alimony or child support but other then that you would be doing more harm then good.

Mediation is about co operation not about winning or losing. You need to remain calm and work through problems to solve them not try to bully the other party.

No, any threats will only go bad against you, and are basically illegal. If you have proof, real proof, of her hiding income. Use it at the divorce proceedings, then report her to IRS after.

If she does not want to go to mediation, just go forward with divorce and let judge either order mediation or make rulings