This is the main thing I'm worried about. I don't have any psychology or neuropsychology qualifications, so I need to back up my work with studies and so on.
I need to make money so I can fund further research and development.
You are correct. IQ is a flawed system, but it is widely used and can give a general measure of intelligence. I tested very highly on IQ tests, but did quite poorly on psychometric testing for workplaces and organisations until I started studying psychometrics.
Gardner's multiple intelligence model shows that intelligence is much more complex than was believed and that there are many different kind of intelligences, some of which do not correlate or compliment each other.
N-back however increases short-term memory, attention, focus and inhibition control, all of which are the underlying basis for higher order intelligences (logical, mathematical, musical and so on). Short term memory is like a computers RAM. A larger capacity can offset weaknesses in CPU, Video memory and processes allowing a relatively weak system perform at a higher effective operational level.
There are a few.
Kearney-Ramos, TE, Fausett, JS, Gess, JL, Reno, A, Peraza, J, Kilts, CD, & James, GA 2014, 'Merging clinical neuropsychology and functional neuroimaging to evaluate the construct validity and neural network engagement of the n-back task', Journal of the International Neuropsychological Society, vol. 20, no. 7, pp. 736-750.
''The n-back test and the attentional network task as measures of child neuropsychological development in epidemiological studies': Correction to Forns et al. (2014)', 2014, Neuropsychology, vol. 28, no. 4, p. 529.
Jaeggi, SM, Buschkuehl, M, Perrig, WJ, & Meier, B 2010, 'The concurrent validity of the N-back task as a working memory measure', Memory, vol. 18, no. 4, pp. 394-412.