Can an employer for you to work 7 days a week for an extended period of time?

Messrs Neel and Starke nail it. No law on this, but *most* eployees are supposed to get time and a half after 40 hours in a week.

Is this a retaliatory action by my employer ?

Retaliation for doing what? To be actionable the retaliation has to be because the employee engaged in "protected activity." You don't mention...

Since I was a salaried employee shouldn't I still receive my full paycheck on my next payday for the time I already worked?

Have your severance agreement reviewed by a professional. This sounds fishy. Make sure you are not being taken advantage of.

If I earned $500 after taxes for working 40 hours(1 week) at a company, shouldn't it be $1000 after taxes for 80 hours(2 weeks)?

Talk to hr or your payroll dept. They should be able to explain it to you. If not, speak to accountant.

Your former boss sounds like a total jerk! If he/she continues with this nonsense, consider contacting the local police to report the harassment.

Can i fight this?

There is definitely something fishy going on. Get a consult with an employment lawyer. Ideally, band together with several of your co-workers and...

Is it legal for me to be singled out and bullied at work?

Mr. Smith nails it. Unfortunately, unless the conduct is because of your age, gender, race, disability, etc., it is probably not illegal.

Can I sue for wrongful termination, slander, and lost/back wages in the case of being let go for an alleged harassment claim?

This is a lousy situation, but it does not seem illegal. An allegation of harassment, even if it is unsubstantiated, is a valid reason to let...

Bad relationship. Time to mourn the loss and move on. Seems unpleasant, but not illegal.

Is this behavior acceptable? Is this sort of discrimination allowed in the work place?

Tone of voice is not a protected class. Sorry.