No, you won’t be moved to the RD pool. But, no one here can tell you for sure whether the late LOR will be considered. Aside from the counselor recommendation, Penn only requires one LOR. When they download your application, if the required LOR is already there then they will likely consider your file complete and ready for review.
One thing I know from having gone through this process twice is they start working through applications very quickly, often downloading the application file within 24-48 hours. My child has already been assigned alumni conversations/interviews so application review is underway. Hopefully, your mentor can submit the letter today. Best of luck to you!