It is quite likely that your employer has violated the Fair Labor Standards Act. Generally speaking, your employer must pay you overtime for any week where you worked more than 40 hours. You will need to do some further calculations but this is certainly worth investigating.
Generally speaking, non-compete agreements are enforceable in Pennsylvania. That being said whether a restrictive covenant will be enforced against a particular employee requires an examinaiton of all of the surrounding facts. Given that you would be joining a group of former employees I would fully expect your employer to pursue you.
The important piece of missing information is: what was the "customer related issue" that you discussed with your Manager. If your complaint about the customer could be construed as "protected activity" then it is possible that your Manager retaliated against you.
Having been in this situation in many cases, I would say that if you do not voluntarily speak with the attorney, then you will receive a subpoena to testify. Alas, this is our legal system, not unlike jury duty.
An employee is never required to negotiate for his/her pay. All wages that are due to you should be paid as part of the regular payroll. If you are asked to sign a Release then it should be offered along with additional consideration (i.e. more money). If you decline to sign it you get your wages regardless.
You are obviously aware of the fact that an employer must offer additional consideration (beyond continued employment) in order to bind the employee to a non-compete. It is likely that a Court would require additional consideration for the additional terms as described in your situation. The practical quesiton is what if the employee refuses to sign? Perhaps the employer will then only seek to enforce the orignal document.
The one key item missing from your narrative is: did you complain about the "illegal activity?" If not, then this is a fairly straight-forward case. You exercised your right as an at-will employee and quit. You are entitled to be paid for every day that you worked. If your employer is refusing to pay you then you should contact the Pennsylvania Department of Labor's Wage and Hour Division.
An employer may not retaliate against an employee who takes FMLA time. That being said, the mere fact that an employee is out on FMLA does not preclude the employer from making an otherwise bona fide termination decision. Without knowing any more facts, I would say that the timing of your termination is very suspicious.
You have a valild claim to the bonus money under Pennsylvania's Wage Payment and Collection Law. There will be a factual dispute on whether the bonus should have been in the letter. It will be up to the Judge or jury to determine who is more credible. If you win you would be entitled to the bonus money, plus 25% liquidated damages and the recovery of legal fees.