Must I repeat the Isha prayer that I performed sitting, despite my ability to move the feeding tube and pray standing?

If you pray sitting while able to stand without harm, your prayer is invalid and must be repeated. You were required to remove the impediment to standing. However, if you are unable to do so or fear harm, then there is no blame in praying sitting, for the Prophet, peace and blessings be upon him, said: "Pray standing; if you are unable, then sitting."