Here's the thing. The Barren's is not a desert. The land there is completely liveable even if you just live off the land. And that's what the orcs did. They put too much stock in the trade they did with Theramore and Mulgore, and the resources they...acquired...from Ashenvale. You say that the supplies they lost were events outside of their control. That is not so.
They're living in a world compounded by conflict even without the unshakable Alliance v Horde deal. They had the Silithid to deal with, and the initial Naxxramas assault.if they're capable of gathering supplies for the AQ event they're more than capable of laying in stocks against other situations. I'm not saying that THIS situation is of Garrosh's making, but rather the orcs as a whole, or probably Thrall's.
They had the four years between WC3 and the seven years that WoW has been out, for a total of ELEVEN years to try and develop their own infrastructure, but they haven't. I'm not saying they'd make Durotar or the Barrens a forest, but it could definetly be less...Barren. They've gotten themselves into war after war, conflict after conflict and they never had even the inkling of trying to make themselves self sufficient. And they have the ideal shaman x druid combo to do it if the thought even entered their heads.
You say it was events out of their control? I call crap on that since they've never tried to improve their station other than repeatedly hacking away at Ashenvale and worsening relations with the Night Elves.
The Alliance went through just as much, if not more, against the Scourge as the Horde, but they were able to recover because they've spent time improving their supply situation instead of building more tanks and catapults.
And the biggest rub comes to your last question. If...big IF...Garrosh or any other Horde leader had approached Jaina about interceding on their behalf with the Night elves, they'dve found her receptive, and she'd likely have gone straight to Mal and boom bam bling, Alliance once again extends the hand of friendship.
---------- Post added 2012-06-10 at 01:57 AM ----------
Meh, it was mentioned...or rather the treaty it led to...in the Shattering during the scene in the Cathedral honoring all the Alliance dead.