How should I store 15h cooled and then 2h cooked at 65C white soybeans?
Generally, cooked beans keep best frozen, at least if you're keeping them for more than a day or two. As for what you'd refrigerate them in to save the to use tomorrow, I'd suggest either their own liquid (if you're going to use it in the recipe) or drained (but tightly covered). Alternatively, if they're going to go into some sauce, you could let them marinate in it.

A teaspoon of citric acid (assuming anhydrous, i.e., dry) is a fair bit, especially for 1L. That will be very sour. ⅛ tsp citric acid into 1L water is notably sour.