Is it considered that the worshiper who prayed 'Isha and part of the Tarawih with the first Imam, then another Imam completed the prayer, has prayed with the Imam until he departed, and thus earns the reward of standing in prayer for an entire night, even if he did not pray 11 rak'ahs?

The worshipper is credited with the prayer of an entire night if he completes the prayer with all the imams until they depart, because the multiplicity of imams in a single mosque makes them equivalent to one imam. Therefore, whoever leaves before the last imam is only credited with what he prayed. This falls under the generality of the Prophet's (peace and blessings be upon him) saying: "Whoever prays with the imam until he departs, Allah will record for him the prayer of a night."