Manditory overtime in WI, is it legal?

No, you are not required by the law to work those hours. If this was the case, you could be charged with a crime and prosecuted for refusing. You have every right to refuse. Of course, if the consequences of your choice to refuse is loss of employment, there is no law against that, either. The law is that you be paid for overtime.

Wisconsin does have a 'day of rest' law that requires one consecutive 24 hour period of rest in every 7 consecutive work days, unless the employee voluntarily and in writing expresses that they are willing to work the 7th day.  So being made to work six 10-hour days every week is WELL within the law, and also well within reason. Many businesses in manufacturing and retail have busy periods that can see extended amounts of time with greatly expanded overtime. During my career at USPS, all of the negotiated work rules (with the union) about being able to refuse overtime were not in force from Thanksgiving until the first pay period beginning after Christmas, about a six week period. For years, I worked 10 - 12 hour days, six days a week during this holiday period, and near the end of my career when Amazon thru USPS exploded, I worked every Sunday, too. Several times I worked 80 hours in a week, delivering mail and packages in severe winter conditions at times. I probably averaged, for my 38 year career, in the mid-50s for hours per week, and that was factoring in the 40 hour weeks during vacation or sick leave.  Working 60 a week for two months is nothing.

It's up to the employer.  If you don't like the hours, quit.  Next time, ask at the interview whether overtime is required.  If it is, you don't get to choose how much is acceptable to you.  The employer needs you when he needs you.  Again, if you find the hours too taxing, quit.

You have three different points to address with your question and updates:

1 - you are an employee, and not a slave.  Therefore, you are free to refuse to work the overtime your employer has deemed mandatory and they are free to let you go.  That is how At Will employment works.  As long as they pay overtime in accordance with the law, it is not illegal to require working that overtime as a condition of continued employment.

2 - Per Wisconsin law, vacation pay is NOT required, so there is no minimum.  Should an employer offer vacation pay, they must follow the policies they set forth for it.  If you employer has included a forfeit policy (use it or lose it) as part of the written vacation policy, then that policy is legal.  So your statement that saying it is illegal could very well be incorrect.  You need to reference your company's vacation policy.

3 - You would have to either be deemed to have a disability that your employer would then be required to make a reasonable accommodation, or take FMLA leave to deal with the stress.  FMLA leave is unpaid, so it is possible that you can be forced to either work the overtime or take leave and not get paid at all.

In the end, you need to consider your own work-life balance requirements.  It sounds like you are unhappy and should just find work elsewhere.  The employer does not appear to doing anything illegal, just may not be good practices to maintain employee retention,

Well no, you don't.  Nor do they have to continue to employ you.  You are required to work the hours you are scheduled for or face the consequences.  That simple.  You are wrong about the taxes, because tax rates simply do not allow for the math you put forward.