Can I sue my husband's mistress for slander/defamation of character for something she posted on social media?

A defamation case would be difficult. You would have to pay to retain a lawyer, and also prove damages.

Do I have a case for slander/defamation against a coworker?

As Attorney Vaccaro posted, the only witness would be the inmate so only you can decide whether to file a defamation claim.

Can a landlord hold on to your deposit after 30days of you moving out?

I agree with the detailed posts by my colleagues.

Can I sue a company for instruction me to use their product/service is a way that was risky and caused me to lose money?

it will probably be hard to find the seller and company in order to sue them.

I have been dating a woman for 4.5 years. We recently bought a house together and 10 days after closing she told me to leave

I agree with the posts by my colleagues - you need to hire an attorney.

Can my employer retroactively use vacation/PTO days we had agreed would not be counted as PTO?

I would follow the advice posted by Attorney Spencer.

Unknowingly hitting car in parking lot and leaving.

I would follow the advice posted by my colleagues.

Can I take my ex to court for taking my money?

You can sue since you posted in a comment that there was a written agreement.

This sounds like a scam - a credit company would not be fling felony charges.

Can an employer, first ask for a urinalysis with only speculation? If I was to submit to said ua and fail, I would be terminated

Yes an employer can ask for a random ua test.