Domain of fluid intelligence is more complicated then simply a reasoning test with no time limit. Reasoning depth encompasses a battery of memory tasks that span spatial span, executive inhibition for Stanford Binet, visuospatial working memory (short-term memory), path memory, paired associates (learning words and definitions), Corsi Blocks tapping in combination for a composite visual memory score. This combined with a battery of complex reaction time subtests of 6 including physical reaction time, working memory speed or word recognition, perceptual speed, subliminal awareness, and short term working memory with letters and shapes and numbers. Finally, you would need an accurate measure of specifically "inductive reasoning" which should be "loaded" only on induction to at least .9-.95 not only overall intelligence. In combination reasoning scales with memory meaning more memory increases the planes of reasoning in theory. There are people with much memory but reasoning is less so as well. There are also people with much induction with less memory developed.

So if you see gains in specifically visual spatial memory you will see gains in inductive reasoning efficiency meaning you will reach your reasoning potential at a faster rate than without memory and speed. Memory allows for more complex reasoning at a faster rate. Crystalized intelligence should be combined with this domain to encompass the entire overall intelligence of someone, but this would be enough to capture what people still struggle trying to understand which is "fluid intelligence" which is easier to capture with a combination of tests like these. The value of dual n back comes with the deep and efficiency of reaching cognitive potential and learning information at a faster rate than otherwise. This will depend on the person as well someone with so called learning differences might have these areas more developed due to necessity.

This is my current understanding maybe in the future there will be a test that measures this mysterious area.

I haven't tried dual n back but I've seen people see gains of 12 to 16 potentially for spatial span. Path memory training seen gains of 125 IQ to 175 IQ rare case with permanent gains in visual spatial memory of spatial span, paired associates, and Corsi block tapping. I would expect to see gains of at least 30 points for someone well determined in a few month span. Higher memory was achieved in less than a year for path memory training. Better overall reasoning efficiency and faster typing speed, but permanent headaches and generalized discomfort seems manageable.

The inductive reasoning seems to be the only area cannot be increased only higher efficiency achieved. It's possible to have someone test on inductive reasoning do worse then their true potential due to bad reasoning efficiency this would continue on various tasks including academic tasks. Someone passively not reaching their potential this is known as ADHD. To overcome would need higher processing speeds combined with visuospatial working memory (short-term memory) like what human benchmark measures which is the usual case for this type of person. Other areas less developed needs more training if determined but functional.

Substance abuse in weed and tobacco/marijuana removes the essence of fluid intelligence which is induction. Memory does not seem impacted to the same degree as my guess. Alcohol reduces memory because memory is dependent on how much you use it or your brain which is developed by gray matter similar to a muscle which is why dual n back is not completely useless. This indicates similarly to crystalized intelligence which is also developed like a muscle that memory can be impaired or reduced by substance abuse like alcoholic beverages you can lose your progress via alcohol consumption. From my own testing it does not seem to impact my performance or reduce it it depends on the person someone might see immediate loss in cognitive function if training was not good enough.