to be honest, the entire terminator movie series are squad logical and frankly impossible with that reasoning.
remember, the first terminator movie revolved around kyle reese protecting sarah from the terminator sent back in time to kill sarah connor to prevent the resistance. that did not work. but the terminators could not have known that in the future: there are no time-travel communications.
so that means, that you have a repeating never-ending cycle.
T1 events: skynet sends in 2029 a terminator to 1984. they do it with the intention that john connor never is born. that means if they'd succeed there would be an alternative timeline as of 1984.
Either way, skynet - in 'original' 2029 - gives up their current state in favor of having an alternate history. the 2029 skynet will never know what happened, because when the original terminator is sent to 1984, the time is 1984.
so in 1984, we have a terminator to kill sarah connor. Unfortunately however, this means 2029 skynet's idea never worked. Because if in 1984 terminator would have killed sarah, then there would never have been the need for a terminator to travel through time.
There is no way skynet can know this, so the time goes by like it would in 1984 normally and somewhere in 2029 the events occur when a terminator is sent back.
it's a endless cycle. imho, that is one of the aspects making T1 so damn good.
T2 movie makes less sense when you think about that.
In the same sense, there can be no logic in what we know this far about T5. we'll see what unfolds in the movie.
obviously it's already rediculous that there are 2 t-800's, a t-1000, no kristanna loken terminator, and some john connor terminator lovechild.
Personally, i'm not concerned about the story. Except for T1, and T1 stand-alone, the stories don't make sense. apart from that, i love T1, T2, T3 and even T4 - even though i concider T4 to be quite different and not in the line of T1-3.
the only 'problem' T5 has given me untill now is the visual weirdness that you see in the trailers. i would have expected better CGI, but it seems that they've thrown the CGI budget more into the connorminator.
as for the story in T5; it would be quite a plot twist if skynet actually sends back a terminator because of the connornator demon lovechild; kill john connor so there can never be a connorminator.