Are you aware of the AAP and NHTSA recommendations that kids remain rear facing until a minimum of age two? At 18 months your child's spine is nowhere near mature enough to stand up to the force of a crash - he's five times more likely to be killed in a crash than if he were rear facing for another couple years.
I would strongly, strongly urge you to take him out of the Nautilus and put him back in a rear facing seat immediately.
As for the car seat being able to touch the back of the vehicle seat, most vehicles made in the last 5 years or so have advanced airbag sensors that do not allow for anything to touch the back of the passenger seat, you'll need to check your Kia owner's manual for specific information regarding that.
If it's in your budget, 2 Radians is a good idea I think to start, one FF for the 3 year old, another RF for the 1 year old, and then use your existing infant seat RF for the new baby. When the seat is at its expiration or the baby is outgrowing it you can re-evaluate if you want to move everyone up a seat and get something different for the oldest or just get another seat for the youngest.