i answered the first one, because 10 and 11 jump shot is much closer then 10 and 15.
so if you start rather close, but the decay is slower, so the first player at the 3pt line is (just guessing the numbers) 8 js at the 3pts, and the second is more about 7. thats what my experience tell me.
if the starting difference is higher, its harder to guess. maybe the 15 JS 10 JR is about 9 JS from 3pt, and the second is 8-9
or maybe the first one is already 7-8 and the second one 8-9...
its improbable that you can assume the better 3pt shooter simply by adding JS+JR and compare... better JR will overcome in the long ranges, the decay is most likely exponential, and for large numbers, the decay rate will influence more than the stat you begin with.
so if start stats is close, over distance, the JR will be more important(even for equal differences) , but for a very big difference in the JS stats, i need to estimate the decay rate to determine "is the 3pt line long enough"... which is much harder to do
hope i didn't confuse you too much :)