You would have to first challenge the agreement itself, alleging fraud and or misrepresentation in their obtaining the agreement.
The problem I see though is that, even if you challenged the agreement (and returned any severance that was given to you in return for the agreement), you'd have to establish a legitimate basis for a lawsuit. In an "at will" employment situation, it is very difficult to do. If you felt, before this letter, that the employer could reasonably argue that they were reorganizing and they, in fact, are getting rid of that position, then the letter doesn't really alter that truth. It's just an additional fact, not a contrary fact.
If, however, they did not eliminate the position but just replaced you without someone, the next question is one of "why." You stated that you were on Oregon's leave act maternity leave, and so you'd have the ability to allege discrimination based on the use of that right. You also mentioned something akin to retaliation, by stating that you complained to them about hiring practices that you were not willing to engage in. Finally, you mentioned the FLSA and I don't know if this relates to the hiring practices or if you had your own wage and hour issues that needed to be resolved.
Only you can really weight the potential of those claims against the employer though, as compared with the expense of challenging the validity of this agreement that you signed, knowing that you were using medical leave and knowing that you'd made the complaints about the illegal practices to them already. A court could reasonably hold that you knowingly entered into the agreement because you were aware of sufficient facts that gave you notice of potential claims, and that their withholding of the letter doesn't amount to fraud because they had no obligation to give you an actual reason for their decision to eliminate the position.
On these facts alone, I would consider this a very tenuous claim and if you received a sizable severance, even more so.