yes I just enter my own passport details for the OBJ seat.

any answers to this?

Yes an underseat cabin bag comes with the (OBJ) seat.

So do you book the extra seat on same reservation or separate booking?

Also if booking two passengers and one extra seat do I book all 3 on one reservation or?

Yes all on the same booking. We fly as a couple and like to block the middle seat. Simply add a third ticket with the same name as you but with “OBJ” before the first name and “OBJ” before the surname. You need to pay the seat reservation fee for that third ticket to ensure it’s next to your own seat but just pay the bare minimum, ie. no bags (checked or cabin).

To be clear, can you have 2 people, book a 3rd "extra" seat, but not purchase advanced seat assignment? And then would they actually make sure one of the people has an empty seat beside them? Or are you required to pay for preassigned seating? I would guess there's no guarantee of being seated in the same row with a seat in between but one person would have an extra seat.

You pay for the seat assignment. We usually book row 1 D/E/F, the E seat being the "OBJ" boarding pass.

Flew back from MLA last week with an extra seat booked, I asked the gate agent ahead of time who let the dispatcher know, when it came to boarding both passes were scanned.