Is it permissible to invoke blessings upon the Prophet (peace be upon him) or seek forgiveness while descending for prostration, or when rising for the third rak'ah after the tashahhud in the second rak'ah, with the intention of maintaining khushu' (devotional humility) in prayer?

The prescribed remembrance during descending for prostration or rising is the takbir (saying "Allahu Akbar"), and it is recommended that it occurs between the beginning and end of the movement. If the worshipper does this, there is no room for any other remembrance. If one intentionally abandons the takbir and says istighfar (seeking forgiveness) or salat 'ala al-Nabi (sending blessings upon the Prophet), their prayer is invalid according to the Hanbalis. If one says the takbir during any part of the movement, that is sufficient, and if they add another remembrance, such as salat 'ala al-Nabi or istighfar, their prayer is not invalidated, but it is better to omit it. The best and most complete way is to follow the Prophet, peace and blessings be upon him, in his sayings and actions to achieve khushu' (devotion and humility).