Q: What if somebody forgot to recite dua-e-qunoot in witr prayer?
Answer: If the person doesn’t know Dua-e-Qunoot, they can recite any other Dua instead. If the person knows Dua-e-Qunoot, but still recites another Surah/Dua on purpose then they should do Sajdah-Sahoo. If they forgot to do Sajda-Sahoo, they can repeat witr prayer while there is time for it.
Q: If one prays Isha, then decides to recite Quran later in the night, can they pray tahyatal wuzu after doing wuzu and then should they also repeat Witr after reciting the Quran? (As it is said we must not do sajdah after Witr prayer)
Answer: It is not required to pray Witr again. Once in the night is sufficient.