It looks like this booster in particular doesn't require the top tether but most covertibles will - just a heads up for whoever comes across this thread.Sorry, it's the Everett NXT. Uses the two lower latch connectors and no top strap so it won't smash your headrest.
The backless is purely a question of how the shoulder belt crosses your kids shoulder. If the booster base is high enough that it allows the shoulder strap to cut across the clavicle and not the neck, you can technically ditch the back.I have about 2" until roofline at this point and my son is sitting at just under 44" tall. He could technically go backless, but I hadn't planned on getting him there until he was at 49". 49 is the cutoff to move to backless.