It'll be September I am sure; they'll want to get back on schedule. We've had individual models released in October before, and it never stopped them having a September release the next year. There's the possibility of a split release like last time, but I'm 99% certain we will see at least two models in September.
The short lifespan of the 12 doesn't mean much, it won't disappear and the phone won't magically become worse with the release of the 12S.