Did this very thing on a booking a few weeks back, so here is my experience:

1 - Book your revenue ticket first and note the PNR. When you call in to book the IKK, they will ask for the revenue PNR to reference in the IKK booking. As long as you are on the same flight, it should be fine. Also they will cross reference both the PNRs so that if there is IRROPS, they know both bookings are traveling as a group.
2 - You have to do seat selection for the two bookings separately, with the two different PNRs.
3 - Yeah no access to SS for reward booking travellers