Is it permissible to pray behind an imam who makes mistakes in diacritics (tashkeel) and tajweed, while the worshiper is superior to him in memorization and proficiency? And is the worshiper sinful if he deliberately delays in order to lead the prayer, so as to avoid being led by someone less competent than him?

If the Imam does not make an obvious grammatical error (laḥn jalī) in Al-Fatihah that alters the meaning, such as pronouncing the letter baa in "al-maghḍūb" with a fatḥah, then the congregational prayer is valid. He should be advised gently and kindly on what he errs in. It is preferable that the one with the best recitation (aqraʾuhum) leads the people. The imamate of someone less superior (mafḍūl) is permissible. If you are more entitled to lead the prayer than him, one of the elders of the mosque should clarify that. Otherwise, pray behind him as long as his prayer is valid.