Isn't that giant thing in the background the key from Dalamud?
Could be an older picture, who knows. Point is that if a design document could be revised and be out of date, so can a pre-rendered picture. Personally I'd put more stock into the design document, it wouldn't be the first time (nor the last) that a picture gets posted in the wrong section of a website.
This whole thread is just conjecture. Neither me nor OP has a solid "Mor Dhona is going to be like ___" to pull out of an official press release because such statements have not been made yet. We're all just guessing.
It's quite possible some web designer just slapped that picture on the website in that section like "we need 3 pictures at the bottom, only got 2 so I'll pick one I find cool, whatever". But if this had been in any way contradictory with the developers' intention or misleading in anyway, at some point or another it would have been removed or replaced. It's been there for a long while already.
The behemoth and icy mountain range are part of recent trailers, they're most certainly still of actuality. The map that was brought up is as old as the first roadmap and Version 2.0 announcement, that's like a year and a half ago and some designs most certainly have changed since (remember how they said at first that cities would be mostly unchanged and in the end they admitted that they ended up adding so much it changes them a lot? yeah....). It may have been part of Coerthas then, but moving an area in a MMORPG is as simple as changing where a zone line leads to. It could very well have moved to being part of Mor Dhona after all.
As for drastic landscape changes, take it with a grain of salt, other parts have changed as well. Camp Tranquil certainly didn't have any marshlands with Adamantoises in it, from what we've seen in the live letter, it does now.
Mor Dhona is a little south of where you circled, I believe. It would be more on the border of the brown area and the green area. They did say each current zone is getting split into 3-5 seperate zones, so it could be a desert, a forest, and an icy area for all we know.
Could be, but I'd guess it's probably the Crystal Tower (unles *gasp* they're both the same!)
Yoshi-P did appear to suggest that the key would be explained with the Labyrinth of Bahamut's quest story. We know that this dungeon and the Crystal Tower are unrelated asides from being both long-term expandable end-game content.
I'm pretty sure Mor Dhona is a snowy region based on the image we saw with Behemoth and the Crystal Tower. When asked if we were getting new zones, Yoshida danced around that question by stating that all zones are practically new and never confirmed we were getting new zones.....something I see no reason to deny and something that was never advertised for ARR. I doubt we're getting new zones outside of the areas that already exist at the moment.