Is saying: "O Allah, bless Muhammad as many times as he has been mentioned by those who remember him," counted as one prayer or by the number of mentions? And is writing a fictional story about a young man who faints and wakes up 23 years later without having aged permissible or forbidden?

The specific wording for sending blessings upon the Prophet mentioned is not found in Islamic Law (Sharia). Nevertheless, there is no harm in using it, though it is preferable to send blessings using the wordings found in the Sunnah.

As for counting a single prayer, some scholars are of the view that the reward is recorded according to the number the one remembering Allah has added the count to. Others, however, believe that adding dhikr (remembrance of Allah) to numbers does not necessitate their multiplication in terms of quantity. So, the saying: "Subhanallah adada khalqihi (Glory be to Allah as many times as His creation)" means that Allah deserves glorification by that number, not that the servant has glorified Him by that amount. If a person performing prayer says "Subhanallah adada khalqihi," he has only glorified Allah once.