Filing a MSPB claim would at the very least put you in front of an Administrative Law Judge, I cannot say whether your claim will prevail but it is...

DO i have a case?

You could try to make a case for disability discrimination but you would run into a problem in that your employer was not aware of your condition...

Do I have to go back to work for the same employer that laid me off one week ago ?

Your unemployment could be challenged for being available for work yet refusing.

Sounds like a wage and hour issue that could be handled on a contingency basis. I would start locally with the New York Dept. of Labor before...

I signed a severance agreement and then was hired as a freelancer a few months later, would that nullify my severance agreement?

It would depend on if the agreement included a clause about you obtaining a new source of income as to whether the agreement is nullified....