Is standing for the opening Takbir (Takbirat al-Ihram) for the latecomer (masbuq) who finds the Imam bowing (raki') or prostrating (sajid) a matter of jurisprudential dispute, especially given what was mentioned in the book "Aqrab al-Masalik" regarding the validity of the latecomer's prayer if he performs the Takbir while descending?

Scholars have differed on the validity of the prayer of a latecomer (masbuq) who completes the opening takbir (takbirat al-ihram) while descending (to rukoo'):

Malikis: The prayer is valid, but they differed on whether this rak'ah counts. Majority of Shafi'is: The prayer is not valid if any part of the opening takbir occurs while not in the standing position. However, some of them said it is valid if the takbir is completed before reaching the limit of rukoo'. Hanbalis: The prayer is not valid as a fard (obligatory) prayer, but it is valid as a nafl (supererogatory) prayer. Hanafis: The prayer is valid if one is closer to standing when performing the opening takbir, and invalid if one is closer to bowing.