The Tendai may indeed be regarded as the mother sect of all later developments of Japanese Buddhism, and Amida Nyorai is even of greater importance in Japanese art than Sakya Muni, the founder of the religion.

A spin-off book series revealed the aliens to be originally from Earth, but it's not canon.

in a flame of zeal severe

A shaftment is the width of the palm and the outstretched thumb.