Is it permissible for someone who has chosen to shorten the temporary madd (lengthening) due to sukoon in prayer to lengthen it by four or six beats in the last verse before bowing, or is it obligatory to stick to shortening?

Tajwid is of two types: obligatory (wajib) and recommended (mustahabb). Observing the Madd (prolongations) and similar rules falls under recommended tajwid. An Imam who varies the Madd in his recitation commits no sin. However, it is preferable for him to observe the rules of the art. If he commits to a specific manner of recitation, he should apply it consistently to all similar instances. This falls under preferability for the perfection of beautifying the recitation, not obligation. The author of "Al-Qawl Al-Sadid fi 'Ilm Al-Tajwid" mentioned that if the first instance of Madd Al-Arid Lil-Sukun is prolonged by a certain measure, the second instance should be prolonged by the same measure. He cited Ibn Al-Jazari's saying: "And the pronunciation in its similar instance is like it."