Civil lawsuits use a balance of probabilities standard. If the IP theft was well conducted, and the patent holder forged a careful back story, then the true inventor loses almost automatically.
Also, you're suggesting that if two criminal activities were well conducted, the inventor will lose a civil case? If this criminal activity is revealed, it invalidates the patent and subjects the executives and lawyers responsible to criminal sanctions and massive civil liabilities, including forfeiting all profits derived from the fraudulent patent and treble damages (i.e., 3x the actual damages the true inventor suffered). It's simply not ever worth the business risk to pursue this path.