Well NI is part of the UK, so I wouldn't think you'll have any problems at all re registration, in fact I'd assume you'd keep the one its on unless you're going to move to a personalised one.
As regards an Indy having done the second service, I'd want to know about that Indy - not heard of that name, but if its in NI that could explain why. How old is the KN, as a 2nd service at an Indy I would think personally is a bit odd, unless its low miles etc and warranty had run out before the service was due, so no need to use an OPC (mind you could use any garage so long as its done to Porsches spec using their parts, but most would tend to use an OPC to avoid warranty claim issues).
Most seem to think air and PASM worth it. Ours has them, and they were 'ideals' when we were looking, and really like the ride and handling. I have only briefly driven a KN on steel ,and that didn't have PASM. Ride seemed okay - guess potentially there's less to go wrong!! What size wheels? That might be a factor in the air/steel debate. Ours is on 21's for summer, and 18's for winter. 21's on steel may be a harder ride. I'm sure someone will come along and answer that one.
Whats the rest of the spec? One thing we haven't got and said we wanted, but deal was too good to miss was a reversing camera. I'm going to tempt fate by saying reversing doesn't worry me too much generally, and i can reverse quite well on mirrors alone, so i have got used to managing without a camera, but on a KN it would be a great help.
We really enjoy the KN, hope you enjoy yours as much (we also came from a 997)