I want to make a ready to dissolve oil (in water) (natural plant oil spary for my garden)?

A possible solution to your problems could be the ever versitile eggs. Eggs are a natural emulsifier, biodegradable, and should only benifit the plants, and not harm them. Depending on your insect repellent, it should prevent the eggs from gaining an odour when stored. Also, because of the simplicity, you could mix it up any time.