Thank you for contacting Rhino Rack USA! Yes, this set up does come with the tie down straps.
Hello,this price is for 2 " S512X extensions, so for 1x purchase you will get 2x mounts.
The crossbars that attach to the rails which run lengthwise from front to back on the roof of the vehicle. I like the j rack with extensions but I would practice lifting the kayak over your head a few times without scratching your car. U might want to look at the side loading racks that slide up the side then on top. Wish I would have thought about it sooner.
I believe you can mount them directly to bars.