"The Selfish Gene" does not argue that genes produce selfish individuals, or that all traits are simple Mendelian traits governed by single genes. You've either

"The Selfish Gene" does not argue that genes produce selfish individuals, or that all traits are simple Mendelian traits governed by single genes. You've either not read TSG or have fundamentally misunderstood it. The still mainstream evolutionary biology it describes does not require any particular relationship between genotype and phenotype, just that one exists. Polygenic traits are just as well described by it as simple Mendelian traits. It also accommodates a range of factors like plasticity, epistasis, and gene by environment interactions that complicate the relationship between genotype and phenotype. Additionally, your understanding of "selfish" as genes which produce defectors in cooperative scenarios is incorrect. "Selfish" is a metaphor to describe the fact that to be favoured by selection genes must contribute to a phenotype that increases their own propagation (to the degree that they are not constrained from doing so, and issues like linkage and pleiotropy aside).