Wow - there's a lot here. I will respond to some of the quicker ones. Others will respond too.
Good for you at already diving into research about HPSP and USUHS.
Browse the many, many medical school threads here on SAF - suggest checking USMA, USNA and USAFA forums.
The mission of SAs is to train and produce junior officers for the warfare communities/branches in their services. Medical doctors are staff officers, Medical Corps. A handful are allowed to go to medical school each year from SAs - read about it on SA websites. No guarantee. If you want to be a doctor above all, and have no interest in serving in a non-medical combat warfare specialty, the SA route may not be for you.
The military gets the majority of its healthcare officers via direct commission programs (the ones you are looking at), not via SA.
You will want to research the required years of obligated service associated with each path.
All military people are subject to the needs of their service. This can and will impact everything from duty stations to residency choices. While military people can express preferences, there is no guarantee.
Marine Corps does not have a Med Corps. They get their care from military treatment facilities (MTF) closest to their duty station, usually Navy. They can be single service or joint MTF, staffed by military healthcare providers from Navy, Army, Air Force.
Since you are interested in service, explore the Public Health Service, USPHS. They are one of the 7 uniformed services, but not one of the 5 armed services. They wear Navy-type uniforms, have ranks, have same pay and benefits. They can also attend USUHS. They are professionals and well-regarded. A small Corps but dedicated!
See:
https://www.usphs.gov/student/
Ensure you are talking to officer recruiters, not enlisted recruiters, preferably officer recruiters knowledgeable about the MD programs.