Ad 1. There are two types of "pause": one when I pause the power plant (energy production), second where I pause the game (go to in-game menu). Those two are completely independent. From other games experience, I expect that pausing the game will also pause (temporary) the power plant.
Ad2. For example I was upgrading my power plant from coal to oil one. I buy some coal, tons of water, then build some coal power plants, then buy some heat vents and build first oil power plant. Finally I buy some oil (and water) and wait till coal power plants will run out of fuel. I have no coal, so I expect that my vents for coal plants will work for nothing, so I buy some oil plants and everything is producing too much heat. Simple calculation says: no possibility to produce so much heat, except coal power plants are still working, producing heat from nothing and not producing energy.
Ad 3. I have a D: disk with GBs of free space (as well C: disk). Should I define the path somewhere?
I am waiting for updates! :)