Is standing upon the pronouncement of the call to prayer (Iqamah) considered a condition or a pillar of prayer, such that the prayer is invalidated without it?

Standing when the prayer is about to begin, before the takbirat al-ihram (opening takbir), is not obligatory. Rather, scholars have differed concerning the recommended time for it. As for standing for the takbirat al-ihram, it is a pillar of the prayer for the obligatory prayers for one who is able, and it is waived for one who is not able.