I run OEC simulation with my live account when I am not sure of the market. I could not cxl orders either and I called them. They said they upgraded last weekend but the issue was fixed. I only had that issue for about an hour. Now all is fine with demo. Live account is great, never any issues. You would think it would be the opposite.
I just realized that commission, interest rate are only obvious costs. the more important one is if the broker front-run the customers, cost of one tick to the customer on each order. For example, several traders have identified and complained about intentional slippages on orders by one broker (sorry I do not name names), the broker's representative and customer service danced around the issue for several month.