If your taking them in a car boot, then the water needs an open top, have someone sit with them on their lap in a deep tub of half a bucket of the water from the tank. Never fully cover them up, as that would be inviting suffocation .
As long as the temp drops very gradually as is not freezing the fish will keep, just make sure you use the water from the tank, as it will be warmer and more balanced than fresh water. In a smaller receptacle on the move etc the water will actually hold a certain temp, or at least the temp of it's surrounding environment.
a few hours but don't try it
Maybe a about 5 or 6 hours, but it depends on the temp of the water before the power went out. If you are worried they might die, then wrap a towel around the tank and make sure you have the lid on the tank. You may be able ot get a genorater, but if the power is only out for a will you should be fine. Good luck with the fish. :)
I'm assuming you have a thermometer to measure the temperature of the tank. If it starts going to the lower seventies and sixties then they might start dying off from suffocation. Fish can only breath in certain temperatures. Also it depends on where you live and what the temperature is there. The last thing to take into account is the size of your tank, it takes a lot of energy to change the temperature of water. If you live somewhere with warm weather and you have a big tank then the temperature will fall pretty slowly. Likewise the opposite is also true.
Depends on the temperature.