Is the supplication (du'a) "Our Lord, if we have a place in Your Jannah, then make us steadfast until we meet You, and if it is otherwise, then guide us, then guide us, then guide us, and purify our hearts, and return us to You a beautiful return" permissible, or is it considered ill-conjecture about Allah?

There is no harm in the supplication: "O Allah, if You have decreed me to be wretched, then erase me and decree me to be felicitous, for You efface what You wish and establish." This is supported by the action of Umar ibn al-Khattab, may Allah be pleased with him. Nevertheless, it is better and more befitting for a servant to use the supplications reported from the Prophet, peace and blessings be upon him, such as asking Allah for Paradise and seeking refuge with Him from Hellfire, and the supplication: "O Turner of hearts, make my heart firm upon Your religion."