I would recommend not putting a border on the back at all. Most people don't do a whole lot of design elements on the back, though some have done some really neat backs! Typically, you would use either all the same fabric for the back, you could also buy a wide backing, or I often use leftover large pieces of yardage from the front fabrics. I've also put extra blocks on the back, extra "bonus" blocks made from cut offs from the front, or test blocks. I would not put anything near the edge where you will be trimming later because yes, often the quilt top can shift from the quilting process, and you will trim off the excess batting and backing after the quilting is complete. You can definitely put your 24in block in the middle of the back and that would be fine.
I've never heard of anyone trying to line up a border on the front and the back - I can't imagine trying! That would be nearly impossible unless you were somehow doing a quilt as you go style, but even then, I suspect it would still be really tricky.
The extra length all around on the back would be determined by who and how the quilting is being done. If done on a longarm, you'd want to check with your longarm quilter to see what they require (some have different requirements for loading). If you're doing it on your dsm, depending on the size of your quilt, you could get away with less, but it's trickier to make sure you line everything up properly if you short yourself on that extra length. You just have to be more fussy about basting your sandwich and making sure everything is very lined up. The extra length just gives you more of a buffer in case of any misalignment.