When would you not want to evolve Q or E as Kha'Zix?

E is mobility, very good if your enemy is grouped, q is more power, good if your enemy is splitting, W is good if you are behind, and or need the slow increase (set up kills for others). R makes you invisible a possible 6 times longer, which is nice in general. ( know knows, maybe your ap then take r, w, first.

Nope.

You'd probably evolve R if you needed to hide more, and W if you wanted more CC.

No, you always want Q and E by lvl 11.