At what point does adult John Connor suddenly stop and think "hmm I sense my mum and my past self are in danger - ill send a terminator back thru time and help them out"?
and exactly how does he gain control over the terminator?
1. If he had experienced a Terminator coming back in time to save his ass, then he would know to send one. And if Sarah had told him that he will send Kyle to save (and nail) her ass, then he knows to send Kyle. You did watch T2, right? Besides, the Resistance discovers that Skynet has sent these Terminators when they capture the Time Displacement Equipment. Kyle talks about that in T1.
2. If I could write how they captured a Terminator, I'd probably go with something along the lines of how they stopped the Sentinels in the Matrix with some kind of electro magnetic pulse. We haven't seen anything yet to suggests that a Terminator wouldn't be affected by an EMP. In fact, the blast from the TX put the T-850 down for several minutes. A similar weapon as that may be used. While the Terminator is down, John knows, from the alternate scene in T2, how to remove a Terminator's CPU. The rest will be history.