Unlikely. While 6 plus owners are likely not get the mini and opt for the air it hardly means it will kill it. Alot of users are going to get the normal Iphone 6 because they think the 6 Plus is too big for a phone. Likely they'll think the Air is too big for a tablet too. So they would go for the mini.
Nope. Still use both my iPhone 6 Plus and my iPad Mini (retina) daily. Don’t see that changing. When I’m ready for a new iPad, it will still, most likely, be the Mini in what ever iteration that is current at the time.