If we've learnt anything from the 'pattern' of Apple keynotes. It's that there is no fixed pattern. Yes, they've tended to have iPhone keynotes in September and iPads in October recently. iPads used to be released in March / April, some Macs at WWDC (I seem to remember), some new products have been announced loooong before release, some only a matter of weeks. Heck, the first 4 iPhone generations were released in June/July.
Apple aren't worried about overshadowing their flagship product, last year they announced the Apple Watch at the iPhone event. The one thing we know for certain is that Apple are holding a 'Big Announcement' next week (Siri's words, not mine). I can absolutely see them introducing several new products on 9th September.
There seems very little need to hold an October event. Correct me if I'm wrong, but aren't the El Capitain beta releases keeping pace with the iOS 9 ones? Doesn't that suggest a concurrent release of both?
I trust Apple to know and understand the market for their own devices better than anyone. If they think there's market for a larger iPad with stylus support, then they'll make it work. For those that don't see the point of this rumoured iPad pro/plus, then you're clearly not their target market.