Regarding your first point, researchers from a major Israeli university developed a mathematical model to predict the future carrier rate of the Tay Sachs’ gene in the case that no two carriers ever marry. The result was that yes, the carrier rate will increase , but will take at least 500 years for it to be worth any consideration, and even then the effect will be miniscule.
This is correct. Genetic testing does not increase the carrier rate enough to have a significant effect on genetic disease.
Even if it did, having one recessive gene often confers health benefits.
Carriers of Cystic Fibrosis are more resistant to cholera, typhoid, and tuberculosis.
Carriers of Tay-Sachs and muscular dystrophy have a higher IQ.
Carriers of some genes for deafness are more resistant to bacterial infections and have better cell repair.