You can use the back seats with the Ralleye's...the tail strap and shoulder straps can be disconnected. You can stow the shoulder straps by shoving them under the front seat, you'll understand if you get them.
But if you have kids back there...they will inevtibly play with the tail straps. Probably not a huge issue. Until someone with muddy shoes gets into you back seat and gets mud all over your shoulder straps.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red