In D2 they specifically patched up the multishot spell for the amazon simply because she could do exactly that, and it was too OP. I have a feeling they might do the same here, but then again it is a completely new game so what was OP in D2 might not be here.
We will just have to wait and see
edit: Oh and as for the plague of toads, I believe I saw someone use them at point blank range and it insta-killed the zombie, so I believe it can stack. The difference here is that it's only 3 toads as opposed to 20 multishot arrows.