I was given a RE-3P code for being found unfit by PEB. This should supersede the RE-3O, but either way your time in the USMC is over for good. If found fit, the RE-3O would stick.
Severance pay is a HQMC decision. However, since the PEB (SECNAV authority) will direct your discharge to HQMC, I would suspect the medical severance pay would take place. If you're medically retired, you won't get seps pay, but IMO the lifetime benefits would out weigh a one-time check, which will be repaid through future VA disability payments until the DSP is repaid. Especially if you qualify for CRSC.
Also, if your EAS becomes an issue know this federal statue which allows med hold, if your career planner or command tells you your EAS is the only way.
"An enlisted member of an armed force on active duty whose term of enlistment expires while he is suffering from disease or injury incident to service and not due to his misconduct, and who needs medical care or hospitalization, may be retained on active duty, with his consent, until he recovers to the extent that he is able to meet the physical requirements for reenlistment, or it is determined that recovery to that extent is impossible."
https://www.law.cornell.edu/uscode/text/10/507
Also MARCORPSEPMAN aka MCO 1900.16
"Medical Hold. The medical status of a Marine to remain on active duty 60 or more days beyond EAS to receive medical treatment for service connected injuries, illnesses and/or diseases. Retention 60 days beyond EAS requires the commander’s approval of TLD based upon an AMEB(R), which clearly indicates medical conditions, limitations and prognosis for recovery or referral of the Marine into the Disability Evaluation System (DES) via the PEB and that status reported in the MCTFS. Marines held 60 or more days beyond EAS in a convenience of the government medical hold status (CofGM) not in a valid TLD status in the MCTFS will have an EAS established by CMC (MMSR-4). Retention beyond original EAS for a second period of TLD requires CMC (MMSR-4) approval of a MEB(R) with commander’s non-medical assessment or referral of the Marine into the IDES and that status reported in the MCTFS."