Is it permissible to supplicate with the formula: "Oh Allah, if You know that I will be happy with my husband, and that You, O Allah, have not destined separation for me from him, then grant me righteous offspring from him and hasten it for me. And if You have decreed separation for me, then do not grant me offspring from him"?

There is no harm in the aforementioned supplication, though it is preferable to supplicate with the reported (from the Sunnah) supplications and not to explicitly state a request whose outcome is unknown, for Allah knows the ultimate consequences of matters. Along with supplication, one must entrust the matter to Allah Almighty, truly rely on Him, have a good opinion of Him, and be content with His decree. For He, glory be to Him, is the Knower of what is best, as corroborated by His saying, the Most High: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not."