the newer seats it doesn't matter which slots you use for rearfacing, the standards have just changed. For example the meridian tilt and adjust you don't actually have to move the straps in slots cos the whole headrest has multipleadjustments and the straps move up and down with it automatically...there is a shoulder mark on the seat and as long as you keep the shoulders at that level you are fine.

re the back of the front seat impacting, that is why there is a clear zone needed between the car seat and the front seat....if yu look at the installation nstructions for a car seat it specifies you must leave a gap of x cms between the carseat and the frontseat. This allows for movement in a crash situation. But yes you wouldn't want the head above the seat rearfacing, which is highly unlikely to occur in a convertible seat as they designed to last up to four years old. If the seat is forward facing it is okay for the top of the head to be above the top of the seat...the indicator in that situation is if the top of the seat gets to eye level it is time to move up to a bigger seat.