In case you're being serious, this is unrealistic. For one thing, mining will continue to become relatively more capital intensive; rather than the costs of mining being primarily from electricity, they will be from the capital investment in equipment required.
I am being serious, although I am not claiming that this actually will happen. Just that it's one of many possibilities, and we don't really know how funding of network security will play out over the long run.
Your estimates about capital equipment expenditure don't seem right to me. Mining hardware just isn't that expensive, especially once ASIC companies start getting economies of scale. We're very close to the limit of what mining technology can do before it's only a few process nodes behind state of the art.
Currently, cost of mining is going up because it's transitioning from an activity for hobbyists to an activity for professionals who need specialized equipment. There is an equilibrium point at which the cost of slightly faster hardware is not balanced by the increased revenue.
Another thing: Bitcoin mining is not the first industry with the potential for portable units and which produces large amounts of waste heat. To estimate how much heat might be recoverable economically, and what the value might be relative to input energy costs, one could survey existing industries. Anyway this approach is going to be seasonal.
Yes it will be somewhat seasonal. Mining is ideally suited to this though because you don't really need anything beyond a source of electricity (wind, waves, biogas, whatever), some hardware and an internet connection. In particular it means you can extract energy from places where it's uneconomical to get it all the way to the grid.
Also, it's worth remembering that all large electricity grids need load balancing capacity. Today, grid operators have contracts with large consumers of power such that they can remotely shut down or spin up heavy equipment to balance moment-to-moment fluctuations in demand. For example, in the United Kingdom there's a hydropower facility called Dinorwig which pumps water up a mountain or lets it flow down in order to absorb/generate power spikes. Storage heaters and the Economy 7 tariff also incentivise people to use electricity when there's a temporary surplus and avoid it when there's a temporary shortfall.
Mining, being an activity you can switch on or off instantly, is an ideal form of load balancing - if there's a sudden unexpected drop in electricity demand for some reason, or just an anticipated drop at night, mining hardware can switch on and absorb the excess capacity reducing the need for power shedding or inefficient cooldowns of gas turbines.
The time when it drops to zero is not significant. The lower the rewards are, the easier attacks get, continuously. Pointing out that the very tail end of the exponential decay is well in the future is a red herring. We know that the current rate of 10% or so annually is probably more than the long-term rate required. In around 7 years, after the next two rewards halvings, the reward rate will be more like 2% of the monetary base annually, without transaction fees. At that point, I would be seriously worried if that were the only direct incentive to miners.
You don't know what will happen to the exchange rate in future. Currently the FX rate is quite clearly the dominating factor in how much mining is done. An inflation of 0.5 BTC per block could be more than enough if each coin is worth thousands of dollars.