I'd get an additional chameleon skink. 6 will on average do 2 poison attacks, that's more reliable to take out any potential bolt throwers. If you know he doesn't have one I'd probably just use the points else where.
That's not true. Chameleons can be good for other things than just shooting. If you get first turn, then they'll be a good match blocker, also perfect for forcing the witch elves to overrun in a direction of your choosing.
Depends if he have other units you can target. Personally I'd still have a few more though if that's the case. One thing about chameleons is that they'll mess up your opponents deployment. If you can dictate how your opponent deploys it's they've done a good job for something that cheap. Just consider what you'll want to use then for. Shooting big blocks of units isn't worth it.
He runs tiny blocks of witch elves which are easy to pick off with shooting attacks. H'e also usually got his General on a Cold One. Either of these viable targets?