This is a really tough situation, and to be honest, there probably is not going to be an ideal legal remedy for you as most of what you describe is nasty office politics but not in violation of any law. Reporting the embezzlement would only be considered a form of legally protected activity (thus protecting you from retaliation) if you reported it to a government agency such as the IRS. An internal report of embezzlement is not protected, and therefore retaliation for it does not violate the law.
In general employers have no legal obligation to pay severance except in certain limited and seemingly inapplicable circumstances involving mass layoffs. In all other circumstances, an employer's sole motivation for offering severance is to obtain a waiver of that employee's right to sue. Thus, the amount of severance an employer is willing to offer (if any) is going to correspond to the perceived legal exposure of a lawsuit.
Here, for the reasons noted above, it would not seem as though there is any legal basis to sue your employer unless you reported the embezzlement to the IRS or another government agency and you could prove that such report was the motivation for retaliation. Otherwise, this is a case of nasty office politics that do not violate any law. So, you can certainly demand severance in exchange for a waiver of your legal right to sue, and it is definitely possible that your employer will be willing to offer something just to avoid against the possibility of a lawsuit, but since any basis for actual liability is doubtful, your employer if they are well advised probably won't be willing to offer that much.
I know this is not what you were hoping to hear, but I am trying to be honest and realistic with you so that you can move forward appropriately. I would be doing you a disservice by suggesting that you had tremendous leverage here when you do not.
I hope that you find this information helpful if not particularly encouraging. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.