There is no dosage where alcohol is beneficial/useful to your body. Just a dosage where its detrimental effect is somewhat minimal.

There is definitely a healthy/beneficial/useful dosage of sugar. Especially while exercising, sugar is not stored and is used as litteral fuel by the body.

I'd argue that processed or refined or concentrated sugar (not talking about berries or vegetables) is similar to alcohol in being a net negative impact on the body in 99% of cases. Sure if you are otherwise starving then it may have a benefit. But if you trade calories from some real food for calories from sugar, you're worse off.

What’s the beneficial dosage of refined sugar?

It isn't. Everything depends on quantity and quality.

Kefir and kvass - do have alcohol for example. Low ammounts but it is there.

Not to mention dry red wine and such. Drink a bottle day a you will have problems. Drink 100ml every now and then and you will be the same as before.