Being a compact SUV is for sure going to have some little space or interior fitment issues.
But this headrest thing, could and should have been forseen by mazda (simply by using different types of headrest like come euro models) or having back seats that can be pushed back further. Its not due to the size of the CX5, because this problem is not present in a smaller VW Tiguan (to name one).
And its specially annoying because of the great trunk release latch system... The system works great, but the headrest issue kinda ruins it.
I'm only 5'10'' so my seat is not so far back, but the rear headrests still hit the back of my seat when folding them.
Its not a big deal since I rarely fold them down, but I can see this being frustrating for someone who does regularly.
Removing the back headrest is an idea... not a solution... at least it shouldn't need to be.