Diffusal is if you want to play more gank oriented. Manta is questionable, it used to be the natural build up from Diffusal, but I haven't tried it since the change in 6.6x. Butterfly if you somehow manage to farm the gold, bottle for rune whoring/regen, 2-3 wraiths early. Maelstrom->Mjollnir is if you want to be more of a carry. Maelstrom and Diffusal are both good items for her, just dependent on what you want to do more. I personally prefer Diffusal, but its up to you.

I prefer Deso to maelstrom for a carryish mirana. Mjollnir is great if you have a tanky guy like axe up front and if you can farm like a freaking beast for it very quickly.