1)
Why does it say "me'Hara Osach"?
1.
Radak: It is like me'Hara Lach (to harm you).
2.
Malbim: I would have killed Tzadik (you) with Rasha (Naval). Hashem caused that no harm come to you.
2)
Why is it written va'Tavosi, and we pronounce it va'Tavos?
1.
Radak: Both of these have two signs of feminine - past (Bas) and future (Tavo). This shows that she was zealous.