Is it valid to fast Monday if one intended to fast on Sunday morning, then remembered the intention to fast on Monday morning, and is the reward diminished if one fasts on Monday and intentionally omits Thursday?

The intention for fasting on Monday, if made during Sunday's daytime, is not considered valid by those who stipulate that the intention must be bayyāt (made during the night), which means forming it between sunset and dawn. Jurists have differed regarding the stipulation of a bayyāt intention for voluntary fasting: the majority holds that it is not a condition, while the Malikis stipulate it for all fasting. Therefore, your fasting on Monday is sufficient according to the majority. It is better to fast both Monday and Thursday together, because deeds are presented to Allah on these two days, and the Prophet loved that his deeds be presented while he was fasting. If you fast only Monday and intentionally omit Thursday, you have missed the reward of fasting Thursday, but there is no sin upon you, and you are hoped to receive the reward for fasting Monday; for each day is an independent act of worship.