re: Terminator 5 (Reboot)
Haven't seen Southland Tales. May just do that to see if I get it.
Anyway... you are right. A grunt "terminator" doesn't know all. Which is also why you cannot really trust what the terminator in T3 is telling John and Kate about time travel theory. It doesn't know more than what it has on file and unlike Reese who acknowledged he didn't know, the terminator would recite it as FACT, even when it isn't.
But you are attacking this story-line from pieces that weren't in any of the movies, so, it is just as little fact as the cut scenes. But you seem to be close to the production of T4 and a potential T5 and could know where things are going, but for the rest of us, until it's up on screen it is not fact: canon.
Also, think about it. What actually happens in T2 - if you forget T3 for a moment and runs with the unchangeable past and future concept? Sarah, John and the Terminator run through hoops and does a lot of damage, but in the theatrical cut, there is no real conclusion as to whether they actually changed anything or just did exactly as they were supposed to. What facts do we know? The terminator talks about the history of Cyberdyne and the creation of Skynet and the man most mainly responsible, being Dyson. Well, they get to Dyson and he helps them break in and destroy the chip and arm and the research and half of the Cyberdyne building. But then... did that actually change or stop anything? In fact, it could be a big fat NO! And here's why.
Cyberdyne is already established as a company willing to lie. It lies to its employees, it lies to everyone else and as a result: Sarah Connor is in a mental institution. So, is it so hard to assume that the company, who's on the brink of a major discovery doesn't have back-ups of all Dyson's research stored somewhere safe? That they are not willing to lie about the terrorist incident committed by the Connors and their main employee - lying about him being even killed - to save their stock value and assure their shareholders that things are not as bad as rumored and even going to great lengths to compensate Dyson's widow or putting a gag order on her regarding talking about her husband... and just keep putting his name on the final development of the chip system that eventually creates Skynet. They wouldn't be dumb enough, now that one main employee got blown up in the attempt to eradicate the work, to want to give the Connors any chance of finding the real guy responsible for the further development. + if they lie to the public... they'd probably not have written documents revealing the truth, so again: Skynet's knowledge is limited to what is fed to it by Cyberdyne.
But, acknowledging T3 and that T2 altered things. It can easily still work, because, again, the terminator spewing theory as fact really doesn't know, it just doesn't have the capacity to realize it. So, things can be altered AND can be stopped altogether, so again: nothing is certain... but with all these changes... it simply doesn't link back to T1 in any way - it links back to another '84 incident, as these later movies are in other timelines altogether. And, if you accept the possibility of alternate timelines, there can even exist timelines where there is no future war, no terminators and ultimately... no John Connor - in that time line Sarah may have died an old spinster or given birth to a girl or even several kids and not one of them named John. THAT'S the effect of multiple timeline theory. NOTHING is certain, 'cause you can just go back in time and change it, but unlike the fixed timeline theory where you jump back in time to your own past, with multiple timeline theory, not only do you jump in time, you also jump timeline and you never get back to the original timeline and that timeline goes on without you as if nothing even happened.
Which is why that you can do any sort of movie now, because that is what multiple timeline theory allows. But you CAN save the series and include everything from T1 to T4 - you just have to acknowledge what is FACT and what is THEORY and therefore not necessary to adhere to.
There IS no fate but what we make... so technically, there is no end date.
I can accept a movie that includes everything - just loose the comedy and gags - but would prefer something that is a continuation of the T1 timeline.