I would think it very likely there will be some kind of opening before Oct. Cast previews, possibly AP previews, and possible soft opens for everyone. Not info yet as to when, but we do have the opening team currently being recruited. Your late Sep may work out, but we just don't yet.
Sometimes with soft openings it's about being in the right place at the right time. For instance heading there as soon as the park opens guarantees nothing. Some days at random o'clock they may just open up so you could wait all day and get that or get nothing.
They need to divert people to Epcot cause everyone will probably wanna be at Magic Kingdom for the start of their anniversary celebration. They have to soft open up Rat like at least weeks before October 1 to get the word out and people would reserve Epcot as a priority instead of MK.