Nickola,
There is a big difference between sizable limit orders at the open on an individual stock and on an ETF.
IWM Market makers are much more concerned with the aggregate limit orders on the 2,000 component stocks of the index than with the limit orders on IWM itself. Indeed, they can create or delete IWM shares at will and do it all the time, arbitraging with the component stocks or derivative instruments to reflect the closest possible instantaneous value of the basket.
There are dozens of market makers acting on IWM, all arbitraging in real-time to best reflect the closest underlying value of IWM, so it's almost impossible for any of them to "trick" you on limit open orders. They may scalp you 1 or 2 cents at times, but that's about the worst they can do.
Billy