Is praying two rak'ahs for qiyam and nine rak'ahs for witr, with the witr prayer connected with two tashahhuds, considered in accordance with the Sunnah? And is the first tashahhud obligatory or a pillar in this case, and what are the consequences of forgetting it?

Your Witr prayer of nine rak'ahs is a Sunnah and is considered one of the established practices of the Prophet, peace and blessings be upon him. He used to pray nine rak'ahs, not sitting except in the eighth for dhikr and supplication, then he would rise without saluting, then pray the ninth and salute. What you did by performing the prostration of forgetfulness for omitting the tashahhud in the eighth rak'ah is permissible, because, according to the view that this sitting is Sunnah, prostration for omitting Sunnah acts is permissible.