I'm also in a doubt with this JR business. Like you've mentioned, if a guy with, say 3 JR can make many 3s well against an atrocious defender, does that mean the ability to make 3s depends on the defender's defense only? I mean, awful JR...how far is the range with an awful JR? (Oh, and assuming that the attacker has a similar JS lvl).
Also, say 10 JR is sufficient to score a consistent 3s. If so, why bother training JR up to 18 for example? Does that mean that he can shoot consistent 3s from the half court? No, this is certainly not the case (at least I don't think this is the case).
It seems to me that if the ability to make 3s have to do with the lvl of JR vs the opponent's OD, then it only makes sense to train it up high (also assuming that the JS lvl is similar to that of JR).