Can one purchase a Cessna 182JT-A although it has not yet been certified? To the best of my knowledge, Cessna's 182JT-A has not yet been certified. At least I couldn't find on Google any indication that it has. Lots of items that certification is "imminent" but no real facts.

Yet, I see them for sale and YouTube has videos of them flying. Am I correct in believing that it is possible to buy one and then operate it as an experimental aircraft?
They did have an engine out failure about a year ago which set everything back a bit.  They've had it at Osh a couple of years now.  Two years ago they were saying around the end of '14 beginning of '15.  Last summer, they were saying months.  If you're really curious, I would talk to the manufacturer in Kansas.  Dealers really aren't part of the FAA approval process...

I wouldn't read too much into the range calculator.  I would imagine that they would be adjusting numbers numerous times before release as they get more data.