Here is how it generally works with medical retirements and separations: A person's medical records go before a medical board and a disability rating is awarded by the board. 10% means the person is medically separated without retirement. He/she does not get benefits as a retiree. These are generally things like PX privileges, base medical treatment and things like that. He/she will get a separations payment, in my day the formula was base pay times 2, times the number of years in service.
IF the rating is 30%, he/she is listed on the Temporary Disabled Retirement List for three years and has those benefits. In three years you will undergo another medical board.
50% puts you on the Permanent Disabled Retirement List and gives you full retirement benefits permanently. IIRC, you do NOT get retirement pay. You get VA compensation that is not based on rank, military pay or time in service. 100% tops out at a little over $3800/month for a married man/woman.
The Naval Academy is the Navy's service academy. The Marines are a part of the Department of the Navy and any Annapolis grad has the option of volunteering to be a Marine Officer.
Beside the Academies, a person can become an officer with a bachelor's and either ROTC or OTC. There are also enlisted to officer programs, and lest we not forget, warrant officer programs as well.