Sake and wine are mainly used in cooking to add aroma, acidity and sweetness, and to influence how aroma compounds behave and the overall flavour balance of a dish. Their main role is not to increase umami.
However, sake contains umami-related compounds such as glutamate, so adding sake can also add these compounds to a dish. When sake is reduced, these non-volatile compounds may also become more concentrated.
In other words, sake and wine contribute to deliciousness not only through any effects they may have on umami, but also by shaping the overall flavour of a dish.

Reducing fat does not necessarily mean losing umami itself. However, it can change the way aromas are released, as well as mouthfeel, smoothness, and the overall body of a dish.
So rather than thinking, “I’ve reduced the fat, so I need to add more umami,” it may be more useful to first identify what has been lost or changed.
For example:
The flavour lacks depth → Reconsider how umami-rich ingredients are combined, extracted, or concentrated.
The aroma seems weak → Add fresh aromatic elements such as herbs, spices, or citrus at the finishing stage.
The mouthfeel seems thin → Build body or viscosity without relying on fat, for example with gelatin, beans, or vegetable purées.
The dish tastes flat overall → Create contrast with acidity, bitterness, heat, or roasted notes.
In other words, rather than trying to “replace” fat with a single ingredient, it can be helpful to break down the roles that fat was playing in the dish and address each of them in a different way.
For example, in a broth-based dish, you might extract and combine umami from ingredients such as mushrooms, tomatoes, beans, or seafood; use gelatin-rich ingredients to add body where appropriate; and finish with fresh herbs or citrus to add aromatic intensity.

Yes. Although they may seem like contrasting tastes, combinations of umami-rich ingredients and acidity can be found in cuisines around the world.
For example:
Fish sauce + lime — common in Southeast Asian cooking
Anchovies + lemon — found across Mediterranean cooking
Oysters + lemon or vinegar — a classic pairing in European cuisines
Aged cheese + balsamic vinegar — a familiar Italian combination
Tomato + aged cheese — tomato itself provides both glutamate and acidity
This does not necessarily mean that acidity makes umami itself stronger. Rather, acidity can add contrast and brightness to umami-rich foods, making the overall flavour feel more vivid and balanced.
If a dish feels heavy, monotonous, or flat, it may therefore be worth trying a small amount of acidity before simply adding more umami.

I have experimented with cold-water extraction using many different vegetables in Japan. However, I am not yet sure whether I can achieve the same results with the water and ingredients available here.
Water composition can influence extraction. In dashi preparation, soft water is generally considered more suitable for extracting flavour, while harder water may produce different results. The variety and condition of the ingredients can also make a difference.
So I would like to test this locally first and share the results with you afterwards.