Not sure on the reason for KC to being on top.
I haven't used it in quite a while.

The macro, with /assist Player...
Player, in macro terms refers to the current character.
So you're essentially saying assist yourself, which won't change your target.

If you always have the active window/master as your focus.
You'd want to either:

#show
/cast [@focustarget] Whatever

#show
/assist [@focus]
/cast Whatever