Will Allah forgive the sin of someone who said "In the name of Satan"—thinking that the bathroom is the devil's abode and it is forbidden to mention Allah's name therein—then repented and regretted his action?

No matter how great sins may be, Allah forgives them. Allah Almighty has commanded His servants who have been excessive against themselves not to despair of His mercy, for He loves those who repent. Repentance has conditions, which have been explained in Fatwa No. 32521. Increase your استغفار (seeking forgiveness). Do not listen to the whisperings of Satan, for he calls his party to be among the companions of the Blaze. And if you err in speech unintentionally, without intending disbelief, then Allah will pardon you, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are forced to do."