Is it true that joining verses in Hafs' recitation is impermissible, and that it was not transmitted from the Prophet (peace be upon him) except in specific instances?

None of the scholars have stated that it is obligatory to pause at the end of every verse and forbidden to connect some verses with others. Rather, it is well-known that doing so is commendable. Scholars have differed on whether it is commendable to pause at the end of every verse unconditionally, or if it is better to consider the meaning, unless pausing leads to a corrupt meaning, in which case pausing is not commendable. Although pausing at the end of verses is a Prophetic Sunnah, this does not mean that it should be done continuously. Rather, the meaning is often taken into consideration when pausing, even if it is not the end of a verse. Scholars of waqf and ibtida' (pausing and resuming) have explicitly stated that pausing at places where there is a strong connection to what follows is inappropriate, even if it is the end of a verse, so as not to corrupt the meaning.