>a, resources are very limited.
only certain resources, such as wood, are strained. There is a good number of farmers and fishermen who live in settlements, producing food for pirates or traders when they come to port. The rarity of ships keeps many people landborn and having to produce something.
>b, the people that product nothing themselves live forever
The oath is very well known and any man afraid of death can take it, meaning that a farmer who doesn't want to die can swear himself away and continue raising live stock until crusaders visit his home and take him away. The threat of crusaders visiting home towns is great in the core (which relies on agricultural and fishing trade) that many tend to stay away from the oath, but it's not to say that they're not there.
>c, 'society' is constantly moving outward, leaving nothing behind in their wake.
With the immortals able to reproduce, they are simply sending their following generations out further while they stay in the core, there is still plenty going on there, where everything is mapped and merchants flock in from the fringe to sell goods to wealthy barons.
>d, when these worthless non-producers die, they return to the middle of this ring of nothing, to return to a land that is most likely already stripped of anything of value and no way to leave.
again, it's not stripped down to nothing. Most likely their own village is still there and if they're born in a village that promotes piracy, they will likely find a ship out. If not, they will have to wait for a ship to come by or try and swim/raft to the nearest port. they have a while to try at least.