Q1) In Jodo-shu, what is the purpose of the Nembutsu? How does it differ from its purpose in Jodo Shin-shu? Is it also simply an expression of gratitude for the gift of Shinjin [faith], or is there more of a "practice" in Jodo-shu?
A1) In Jodo-shu, the purpose of Nembutsu is to "ojo" (to be born in the Pure Land).
Amida Buddha says, "Call my name. Chant 'Namu Amida Butsu,' and I will come and take you to my Pure Land when you leave your body." And we answer, "Yes, I understand. Namu Amida Butsu. Please take me to your Pure Land." Thus, the Nembutsu is a response to Amida's call and at the same time the only means for us to ojo. This is the Nembutsu of Jodo-shu.
Jodo Shin-shu says, "It's certain that you will ojo because Amida's Vow has already been accomplished. Learn and understand this, and your Nembutsu will become an expression of gratitude for your salvation."
This difference comes from the following: Honen Shonin once said, "Amida's Vow can be achieved by profound faith." He also said in another sermon, "If you continue chanting Nembutsu, you will have "sanjin" (three minds [three kinds of attitudes]) in the meantime."
The former teaching developed into Shin-shu, and the latter into Jodo-shu. Shin-shu emphasizes profound faith, and Jodo-shu emphasizes practice. This is my understanding.
But Shinran Shonin might laugh at us, saying, "What a misunderstanding! Master Honen's teaching and mine are not two, they are exactly the same." Thus, perhaps we have become lost in a deep forest of doctrines and sectarianism.
Q2) I've heard it said in (less than authoritative places) that Honen Shonin regarded Master Zendo as an "incarnation" of Amida Buddha. Is this at all true? If so, can Master Zendo be regarded as a Nirmanakaya of Amida Buddha [in Jodo-shu]?
A2) Yes, that is true. Honen Shonin said, "Master Zendo was an incarnation of Amida." However, we look upon Zendo as a great master of Chinese Jodo Buddhism and the master of Honen Shonin, not as a buddha. We think Master Zendo not "as" an incarnation" but "as if" he were an incarnation.