As far as the CPU doing the resigns. With auto pilot off and resign set to on. The CPU will resign their own free agents in-season through-out the season. If you want to control all the resigns for every team then you need to make sure auto pilot is off and resigns are set to off and not auto.
And yes you got the trade method right. But you can do whatever you want as far as who you want to trade for. It does not have to be the CPU's trade block players only. But using the random number method does make it cool and variable.