you definitely know what you are talking about. I think your list is pretty complete and accurate....and i too never liked Raktima's voice too much. Thought Sachali should have gone further than she did.
With regards to Nihira and Hema having the best base...i think i might have to say nihira and sharib. Hemachandra's voice sounds naturally great...but somehow the classical traning bit doesn't come through. It is very evident when you listen to singers which school of music they come from...in carnatic singers you look for what is called "gamakkam" in their voice, and in hindustani music you look for the "murki's", the "taan" and the "alaaps".....while I find the latter quite evident in Nihira's and Sharibs singing...no trace of carnatic training in Henmachandra's voice.