Minecraft – How to increase the knockback of a slime in minecraft

Your issue here is that your values are not integers. Because enchantment level and id are both stored as shorts, you must add an 's' after them. Thus, the working command would be:
/testfor @p[r=5] {Inventory:[{tag:{ench:[{id:16s,lvl:1s}]}}]}

There's no way to use /blockdata in a radius.

I once made a command system to do something similar, but that's way too much effort and especially too much to process (I get about 5tps with it on a relatively good PC).

So I would recommend you to just execute the command multiple times, each with different relative coordinates, that should be the easiest solution for your case.