As for the former:

The variations in the wording of remembrances and supplications are like the variations in Quranic wording, such as ta‘lamūna (“you know”) and ya‘lamūna (“they know”); bā‘idū and ba‘‘idū (two forms meaning “make distant”); and arjulakum and arjulikum (two grammatical forms of “your feet”). It is well known that Muslims agree that combining these variant readings is not recommended for someone reciting during prayer, or outside prayer for worship and reflection. Some reciters combine them on certain occasions only to test their memorization of the variants and their ability to distinguish the readings. This practice has been a subject of discussion. As for combining them in any prescribed and enjoined recitation, this is not prescribed, by agreement of the Muslims. Rather, one may choose between these variants, and it is good to recite one on one occasion and another on another occasion.

The same applies to remembrances: it is good to say “greatly” on one occasion and “grievously” on another. Likewise, it is good to say “upon the family of Muhammad” on one occasion and “upon his wives and his descendants” on another.

Similarly, in the tashahhud, it is good to recite the version transmitted by Ibn Mas‘ūd on one occasion, that transmitted by Ibn ‘Abbās on another, and that transmitted by ‘Umar on another. For the opening supplication, it is good to use the version transmitted by ‘Umar on one occasion, that transmitted by ‘Alī on another, and that transmitted by Abū Hurayrah on another, and so forth.

More than one scholar, including al-Shāfi‘ī and others, has supported the permissibility of the various transmitted forms of the tashahhud and similar formulas by citing the hadith in the Sahīh collections reported from