Is an increase in the natural length of a vowel considered a clear mistake (laḥn jalī)? Is an increase in the necessary length of a vowel—such as extending it to seven or eight counts—forbidden? What is the ruling on listening to a reciter who extends the vowels?

The natural Madd (prolongation) is two counts, and exceeding it or the Madd Lazim (necessary prolongation) is a minor لحن (error in recitation) that does not reach the level of prohibition.

It is disliked for a person who makes such minor errors (Lahhan) to lead the prayer, if these errors do not change the meaning. His prayer is valid for those who do not make such errors. However, if the error changes the meaning outside of Surah Al-Fatihah, the validity of the prayer is not compromised unless the error was made intentionally, in which case both their prayers become invalid.

It is obligatory to observe the rules of Tajweed in matters that alter the structure [of the word] or corrupt the meaning, and it is recommendable in matters that beautify the pronunciation.

Tajweed is a matter of beautifying one's voice with the Quran and is not obligatory; there is no harm in not reciting with it.

There is no harm in listening to someone who recites the Quran in this manner. However, it is preferable to listen to someone who recites proficiently, like the recitation of Ibn Mas'ud.