I went through this process myself (D3 runner).
Technically, it is not a redshirt if you just don't join the roster for the outdoor season. You retain your eligibility, and there is no paperwork or anything to figure out. Medical redshirts are retroactive, and are only necessary if you competed in meets in the beginning of the season. As long as you don't compete for the team and are not declared on the roster, you don't need a redshirt.
Technically, you cannot practice with the team once the first meet is completed. However, this rule is essentially impossible to enforce. We had people who were off the roster practicing with us pretty much every day. If you are caught then you would lose that season of eligibility, but the chance of being caught is so minimal.
You can run meets unattached (you are allowed to do this even if you are on the roster). You cannot travel to said meets on the team bus/van.
Hope this helps.