Is the fast of one who doubted being in a state of janabah (major ritual impurity), and did not confirm it until after the intention to fast and refraining from food and drink, then performed ghusl (ritual bath) with the intention of purifying from janabah and wudu (ablution) for prayer, valid?

Your fasting and prayers are valid. For fasting, purity is not a condition for its validity, and hesitation in breaking the intention of fasting does not invalidate it, according to the preponderant opinion. As for prayer, the Shafi'i school of thought holds that whoever doubts whether what was discharged from them is semen (maniyy) or pre-seminal fluid (madhiy) has the choice between the two. Since you doubted and considered it semen, and then performed ghusl and prayed, we hope that this is sufficient and your prayer is valid. Ghusl al-janabah (ritual bath after sexual impurity) suffices for wudu' (ablution).