In my experience employers will only mediate early with a former employee if the former employee does not have counsel because they know the person on the other side has no idea what the claims might be worth and the employer stands to take advantage of the imbalance of knowledge. The employer will either come in with counsel, will be the company's employment counsel, or send in somebody will clear instructions from counsel how to negotiate.
If you want to know how to value your claims then your best path is to hire counsel. The employer may decline to mediate once you show up with counsel or if you give notice that you will appear with counsel but all that will tell you is that the employer was not going to give you fair value on your claims.

