I think you should rather use 25-30% in your calculations. ASICS are coming fast now, and will skyrocket the difficulty.
Yes, but not monthly.
The difficulty increase in percentage staying the same means that the average increase has been the same.
Lets illustrate this:
Difficulty 10m. For illustration, lets say it is 10th/s (I know that is wrong.)
Hashrate increase of 10%, increase 1m. 1th/s
Same hashrate comes up: 11m, Hashrate 11/th/s with a 1th/s increase. After 2016 blocks, difficulty is increased by 9,09%.
If you suggest a cumulativ increase of additional percentages, you need to think about the costs. 360 million is RIDICULOUS. I have written about this before.
You would have to add the total hashrate of June to accomodate October increases. Just think about that. Then you would do it again, and again. Adding the current total network hashrate several times over, with increasing the increase absolute number every time.
I would agree that a cumulative outlook is nearly impossible.