This organization will accept donations in fiat money and at least guarantees the minimum price on bitcoins. For example, if the organization took donations for the $ 1 million, it can guarantee a price not less than 1/7.5 = $ 0.13 per bitcoin.
It would only guarantee that minimum price if no more Bitcoins were being produced. In reality, to maintain the guarantee you would need additional donations every day or the value of that guarantee would drop as new coins were produced.
Yes, but it's not a problem. There are 7.5 millions of coins today, if this organization will guarantee for example $1 for bitcoin ($7500000 needed) then after 1 month with no donations it will be still able to guarantee 7500000/7716000 = $0.972. I think this is enough to ensure some stability. The fall of three percent per month is very different from a fall by 30 percent in one day like yesterday