Hello T, if I may. I think you are confusing ECN and STP. This happens a lot around here. ECN is Electronic Communications Network, it's basically just the platform that allows you to communicate with the "market", instead of picking up the phone, or being on the floor. STP is how the trade is processed, Straight Through Processing. So nothing touches or modifies the order in anyway.
Now back in the day, STP was a big deal, but it really wasn't. If a trader executed 1k units at x price and x product, if there was no small liquidity, the Broker filled the order with "Inventory" they had on hand, so the matching engine would match size and price to liquidity that the broker had on hand. Even in STP processing, no PB would pick up that size (1k). Now sometimes if there were enough trades, well they got lumped together, and a PB would pick them up, but in general, even though it may have been STP, no spread added, you still would more that likely be executing against the Broker liquidity.
As far as "trading against you" those "offsetting" transactions are performed on a Liquidity Hub far far away and not even connected with the "retail" Hub. The brokers have to be able to do this, if not they would literally cease to exist. If one has 80-100k and can get on a Currenex white label platform, well then you execute against the big boys.
Anyway, ask some questions, I have been behind the curtain enough times to know the ropes, and yes there can be some ugly and nefarious stuff, but for the most part it is not a big conspiracy, ok, now let me take off this foil hat.
The Ever Light Shining VIPER