Can I submit an abstract to present already published work?
I think generally speaking you're thinking about your advisor's point the wrong way round. The point isn't that the journal now owns your work and presentations thereof (re: point 2), it owns the copyright, and the right to distribute their presentation of your work.
Rather the point is that conferences are expected to be new work, ahead of the literature. People don't go to conferences to hear a paper read aloud, they go to hear about new things they can't read in the literature. It's a venue for feedback (which can't be applied to a published article).
I'm not in preventative medicine (but at one time was in a medicine-adjacent field), but have heard this before. Generally, an oral presentation might be the better avenue for work that was just published, but you should ask your advisor about point 1.
To point 3 - no.
To point 4 - no, the managing editor is not the one imposing the &quot;restriction,&quot; if you wanted to ask someone besides your advisor, you would ask a conference organizer or someone else familiar with the conference to learn what level of novelty is expected there.