Is it a condition to recite "Juz' Amma" during prayer to obtain the reward of the "Muqantarun" (those given immense rewards), or can it be recited outside of prayer for those for whom frequent ablution is difficult? And are there other night remembrances that equate to this reward?

Whoever recites a thousand verses will be recorded among the Muqanṭirīn. Some scholars have mentioned that reciting the two parts, 'Amma and Tabārak, fulfills this, and this reward is hoped for whoever rec recites them, even if not in prayer. It is better for one who is keen on the reward to recite them in prayer. There is no other ذكر (remembrance/recitation) that equals this promised reward mentioned in the Ḥadīth.