A launch meetup for the Mini 4? Not going to happen. They'll just appear in the stores in the next few days (they're currently having iOS 9 flashed to them and shipped over to stores)... nobody is going to be in line for it, there will be no shortages. Just like Mini 3. It's nothing new, same thing, they just didn't want the product to stagnate.
There's no stated date. It's however long it takes them to flash with the 9.0 GM and then send them out through the process. I'd guess monday-wednesday.
Typically the earliest day available on the shipping plan...the 16th in this case. That is, unless apple can't prep them fast enough and misses the mark.