Is it permissible for congregants to reproach an Imam who prolonged the recitation in the 'Isha prayer, citing the Hadith "Whoever leads the people in prayer should lighten it," and how was the Messenger's prayer during 'Isha?

It is legislated for the Imam to lighten the burden for the people. The standard for this lightening is what the jurists have determined from the guidance of the Prophet (peace be upon him), which is that it should not compromise the Sunnah of the prayer or its objectives. The usual practice of the Prophet (peace be upon him) was moderation in prayer. He would recite from the long chapters of the Mufassal in Fajr and Dhuhr, from the medium chapters of the Mufassal in Isha, and from the short chapters of the Mufassal in Asr and Maghrib. Sometimes, he would recite from the medium chapters of the Mufassal in Maghrib. The Mufassal begins from Surat Qaf to the end of the Quran and is divided into long, medium, and short sections. An Imam who recites one and a half pages in Isha has not exceeded what is customarily permissible by religious law, because the medium chapters of the Mufassal are not far from this amount.