If left for several days, it will have the same average temperature as the environment, with less extremes. We have not generally recommended this. We prefer to train people to avoid very hot places and use good judgement. The diluent doesn't need to be refrigerated or kept cool. Not necessary, however as in 4 above, when the facilities are available one should use them.
Normal saline is the diluent of choice.
It provides excellent stability, is readily available, easy to make and cheap. We train community animal health workers to make their own diluent with local things like distilled water sold for car batteries and salt.