if and when nitrates hit zero you can either turn it into a ca reactor, take it offline or keep it running, choice is yours!

Run it 24/7 as your tank is always producing nitrates

VP. the recirculating punp has enough pressure to pump it back up to your surface water.

I have a Schuran Jetstream I calcium reactor. I was wondering if it could be converted into a denitrator with adding the sulfer media?

dont see why not.
the korallin unit is basically the same as the korallin calcium reactor minus the probe holder and bubble counter