Am I obligated to offer a sacrifice (hady), and is my Hajj valid after I performed the obligatory Hajj for myself, then performed Hajj again as "mutamatti'" on behalf of my deceased father, and did not offer the sacrifice?

The Qarin (one performing Qiran (definition)) and the Mutamatti' (one performing Tamattu') must offer a sacrificial animal (hady). Their Hajj remains valid even if they intentionally or unintentionally omit the hady, though they incur sin if done intentionally. If the time for slaughtering passes, it must be performed as a make-up (qada'). If one is unable to offer the hady, then they must fast ten days: three days first, then they break their fast for the days of Nahr and Tashreeq, in addition to the days it takes to return to their country, then they fast the remaining seven days.