Hi, sorry it isnt an answer to your question but rather another question. It is a shame that Mental Ray and V ray have moved away from Maya. They were great plugins with great features... I have spent a little time messing with Arnold trying to get a sphere to cast a shadow on another sphere using a central mesh light.. like an eclipse sort of thing..exactly an eclipse.
So it seems that it depends on the light source the kind of shadows you get, also the materials and their settings. You can use the same light source and material but get a million different results depending on your settings. Then there is the render settings which can multiply those results by another million times...
I saw a tutorial on Arnold transparency lighting and shadows. It seems that the Arnold lights are a different animal to the maya light and Arnold treats them differently. There are some Maya lighting settings that dont translate into the Arnold rendereing.
The following is a godd video regarding how arnold and maya lights and shadows differ and how there is different locations for their settings.
https://m.youtube.com/watch?v=YnEfSFPSHH8
Worth watching.