In short, he has no time to screw open the head, he has to send the T-800 through before the T-1000 succeeds; in other words before the timeline collapses and disappears, the T-800 has to go the past to protect him.
Can't agree with this, it's time travel, Connor could wait 30 years, and so long as he sends the Terminator back to the right point, ie the moment the T-1000 shows up in the past, it wont change anything by waiting.
Of course not. This is Quantum Physics folks, Shrodingers cat. At first, especially as the temporal distortions still exist, both time lines - one where the T-1000 kills a young Connor unopposed and the machines win, and one where the machines lose and they send the terminators back through time - the "present" - exist. Sooner rather than later, however, there will be enough observers to have "opened the box" so to speak, and the probability of sending a protector the T-800 back through time becomes too small, and the two simultaneous timelines collapse into only one: the T-1000 kills John Connor unaposed and the machines win, eradicating "the present" where the machines lose.
The T-800 has to be sent through before that happens.