ACL surgery has me facing MAR2. What is the difference between a “Packet MOS” and list of MOSs from which I can select?

An application MOS means that you apply to the MOS and receive an acceptance letter from them.

You don't get to pick your MOS in a MAR2. You list at least three that you are qualified for. If you are qualified for one, the strengths support it, and a training seat is available, then that's what HRC will give you. If not, HRC will give you what it has available. Getting an acceptance letter from an application MOS makes you eligible for that MOS. As long as there is a class seat for that MOS, that's the one HRC will usually grant. Being a non promotable E6, the only application MOS that is open to you SF, Psyops, Recruiter and 25D, all of which you are probably not qualified for.