1- What is incumbent upon me, given that I clipped my nails out of forgetfulness?

If you unknowingly trim your nails during the first ten days of Dhu al-Hijjah while intending to offer a sacrifice, there is no sin upon you. This is based on Allah's saying: (Our Lord, do not impose blame upon us if we have forgotten or erred) and the Prophet's (peace be upon him) saying: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are coerced into."

However, if the trimming occurred while you were in a state of ihram (consecration for Hajj or Umrah), there is no sin upon you, but a fidyah (expiation) is due. This fidyah is: fasting for three days, or feeding six needy persons (half a sa' of food for each), or sacrificing a sheep in Mecca and distributing its meat among the poor of the Haram.