What is the ruling on one who was unable to spend the night (mabit) and stone the Jamarat during the days of Tashreeq? And does he thereby forfeit two obligations or one obligation? And if they are two obligations, does appointing someone else to stone on one's behalf (tawkeel) waive one of the two obligations?

Both staying overnight in Mina and stoning the Jamarat are independent obligations, and abandoning both necessitates two expiatory sacrifices (two "dam"). This is unless there is an excuse with authorization for stoning. Whoever travels after stoning Jamarat Al-Aqaba, and has performed Tawaf and Sa'y, owes an expiatory sacrifice for abandoning the stoning, an expiatory sacrifice for abandoning the overnight stay, and an expiatory sacrifice for abandoning the farewell Tawaf. Abandoning the overnight stay in Mina is not permissible except for those who are considered shepherds and water-carriers. Authorization for stoning is not permissible except for those who are unable to do it themselves.