The problem with most of these spreadsheets is for the later periods. For example, your spreadsheet assumes that the difficulty will increase by 265,669,181,621 on 13-Oct-2014. That means an increase in the network hash rate of 1.9017341e+18 h/s, that's 1,901,734.1 Th/s
I doubt that would be physically possibly even if every foundry on the planet was cranking out ASICs and there was enough labour and resources to assemble mining systems without any delay.
Any thing based on speculation will get more and more innacurate the farther out you speculate. I think it's pretty close for the near term, but there are a lot of factors we can't predict right now.
As far as the difficulty rate increasing exponentially, I don't know how long it can continue. Maybe the growth rate will become more linear soon, even drop once the profitability goes down. If I had a crystal ball I'd be a millionaire. I guess that's why some people will AISCs are selling Meghash contracts, to curb the risk of the unknown for a set profit now.