The most interesting case would be Brinda and Mukta. Brinda never really used a head voice. Her chest voice had a deep range, so above the upper Sa, she just softened her voice to a near falsetto (yes, you can taper your chest voice to just softly touch a higher note and you can go higher to transition into a falsetto, but this will come at the expense of power and manueverability) -- this is a chest voice - falsetto transition and not a mixed / head voice. Mukta however sounded like her head voice WAS in fact her natural "gear", which explains many things.
Now MS amma, if you listen to her sarOja dala nEtri at the UN concert, that final fortissmo to the upper P -- that's purely chest voice. MS amma was familiar with all these techniques, but discouraged changing registers. However, she could also soften her voice if needed to hit high notes without forcing it as well.
MLV, listen from 2:04:00 here :- https://www.youtube.com/watch?v=IdDefl9E7AU&t=7820s
(she hits the upper P at 2:06:45 and goes higher), but there's no belting or shouting - I think right from the beginning MLV knew how to use the mixed voice well, and her lower pitch meant that she also had plenty of upper range in chest voice in her peak days.
Among modern musicians, TMK has spoken to us music club junta about how overcoming register breaks allows the voice to open up to almost 4 octaves. I think he also uses vocal fry for reaching that anumandara panchamam. Abhishek Raghuram also uses a mixed voice for his upper notes (his actual chest voice is a soft baritone). I do not think any of the female CM musicians today use anything other than their chest voices, although the older ladies do use the tapering-falsetto voice. TVG was one person whom I've heard live using multiple registers.
If I had labels to describe how it feels like in your throat - the chest voice would be a ||, the falsetto a () , the head voice a Y, the whistle register a V, and the vocal fry (those low growls) as an IOI.
The actual act of whistling does not involve vocal cords however (that's just blowing air), so it should not be confused with the "whistle register", which actually uses the back of the vocal folds. https://www.youtube.com/watch?v=Kdp4NHWr7G8
-- I will call it "paal cooker" register.
Perhaps next time we should discuss about Thanjavur Kalyanaraman, he was an expert on this.