Manipulations do happen. I think it is obvious.
When markets are not mature yet (this is the case of Bitcoin even though we are getting there...) then it is very possible to manipulate the market.

Actually if the ETF happens eventually, it will be less easy to manipulate, more people involved (= more money), more stable price, more "healthy" price movement... could attract more serious investors, even if the plan of an ETF is in my opinion against the original spirit of Bitcoin.