I derped on her E, forgive me, just woke up then. No base damage, simply impure shots stacks. Still powerful, but MF is immobile without her passive, so that should compensate.
As for what you said, you were planning on giving her minion passive immunity at the start of the game. That'd make her an absolute nightmare to play against unless you were playing Ashe, Caitlyn, or Varus, and perhaps Twitch and Tristana. Basically what would happen is that MF would completely ignore the minion damage and just charge past them, all inning you with little you could do about it.
Her Q's issue is the fact that it's slow to wind up. If you're not building Triforce on MF, your Q will pretty much go unused endgame. Having it function as a basic attack (which it pretty much already is because it applies on hit effects), will make it go much quicker.
Your E is actually pretty problematic. Remember old Tristana? Basically what MF would do is turn on that E, activate her W, and instantly win trades because of huge burst damage and damage over time with grievous wounds. It would further cement highly mobile champions bot lane like Lucian and Graves, because they'd be the only ones to get out of MF's death field. The reason why I chose to put impure shots on her E was so that her Q would home in on champions better.
The biggest issue with MF's Q besides how slow it is is how clunky it is at targeting champions. Unless you've attacked them, and there's a minion that they run behind, your Q bounce isn't going to go after them, it's going to go to another minion. Her Q is supposed to be a bank shot poke ability to hit champs behind minion waves, and it's relegated to a mana intensive farm tool ATM.
Also, MF's E is laughably ignorable ATM. Even if you actually level MF properly and max her E second, it's just bad DPS and a slow. Giving the impure shots passive would improve her Q mechanics, and have the damage ramp up over time.