Wasn't the congregational Qiyam al-Layl mentioned in the evidence without prior arrangement, and wouldn't arranging it on a specific day be contrary to the Sunnah?

In summary, making appointments to observe a specific night [in worship] is not prohibited unless it becomes a regular habit, because the forbidden act is to liken something not legislated (non-Sunnah (definition)) to something legislated (Sunnah). However, prior arrangement with the intention of motivating, invigorating, and reminding people of the sunnah of night prayer is permissible, as long as it does not become a regular practice, thereby resembling a continuous Sunnah. It is permissible to gather for acts of obedience such as supplication, recitation, or standing [in prayer], as long as this does not become a continuous practice, thereby resembling something legislated. Thus, there is a difference between what is adopted as a regular habit and what is not.