Is it true that all disbelievers, such as Jews, Christians, and others, are children of adultery? Has any scholar said this? Is it recommended to curse disbelievers who are hostile to the religion during the Qunut of every prayer, and is this among the virtuous deeds?

It is not correct to say that all disbelievers are children of fornication. Islam has affirmed the marriages of disbelievers and attributed them to their fathers. There is no evidence for the استحباب (desirability/recommendation) of cursing disbelievers in the Qunut of every prayer. Cursing a specific disbeliever during his lifetime is not permissible according to the more preponderant opinion. The Prophet, peace and blessings be upon him, used to perform Qunut and supplicate against disbelievers sometimes during calamities, and then abandon it. Therefore, Qunut is prescribed during calamities as needed, and then abandoned when the cause ceases to exist.