At one extreme is the cost to produce 1 BTC based on electricity. If you assume hardware cost is free (as it is for some miners, who only mine on hardware they already bought for other purposes), the cost to produce a BTC is only the additional electricity expended.
This sets the hard lower limit for value. If value ever gets lower than this point, NO rational miners will mine, and difficulty will plummet.
At the other extreme is the total package cost to buy hardware, pay for it's leased location, electricity, etc, and pay it all off. It's harder to pick an exact point, but i'd say for most people when you can pay off the entire hardware investment plus electricity and opportunity costs within 2 weeks of mining...
This sets the hard upper limit for value. If value gets higher than this point, mining becomes so insanely profitable that even the cautious investors will start buying hardware to mine and difficulty will increase massively.
Reality is that the market adjusts well before these hard limits are ever reached. But you have to realize, there is a wide spread of values between these extremes. Based on current difficulty and hardware costs, the hard lower limit is around $2-3/BTC for many electricity costs, while the hard upper limit is around $80/BTC based on some very rough estimates I just made in my mind. Don't sue me if I am off, the point is there is an upper and lower limit based on difficulty, but it is a wide range and there won't be immediate changes based on difficulty.
No, that doesn't make sense. Miners do not all pay the same costs, they will drop out gradually as price drops/difficulty rises and will start up as price rises/difficulty falls. Some people have very little costs, and if BTC gets low enough difficulty could drop to a point where even CPU mining would make sense again (especially for people who don't pay the electricity costs of the computer they are mining on). I don't understand why you are using the term 'hard limit' for any of this. It is a gradient. As price rises, so will difficulty. If electricity costs fall, difficulty rises (if price stays the same). It is a marginal adjustment and BTC could rise to $100 each and depending on other factors it might not be rational to buy a mining rig.