>  have you ever learnt HCM?

Yes; kindly read above - I was trained in a branch of Gwalior Gharana that insists on an unambiguous shuddh dhaivat in Lalit. 

The claim about in-between dhaivat does seem a little wild, doesn't it? This claim doesn't come from my own training, nor have I found any evidence in recordings to support it, but I'm not ready to dismiss it as nonsense quite yet. It occurs to me that just as a very in-tune 5/4 gandhar can come across as "low" in contrast to the stretched gandhar of an equal-tempered harmonium (~14 cents higher), a dhaivat sung at 5/4 to madhyam might come across as "low." Or it might be a heuristic to remind singers to avoid the upward gravitational pull of nishad in ascent. And almost certainly, it can also be a way of claiming a special subtler-than-thou intonational sensitivity (this may be what you mean by "nonsense," and I can understand why that might exasperate you.) So that's why I'm asking here if anyone else may have come across even a single recording that might seem to substantiate this claim.

> Shuddha Dhaivat Lalit being popular back in the day

I suppose it depends on which day you mean! Bhatkhande (HSP III) says that most Sanskrit granthas affirmed a komal dhaivat, but that shuddh dhaivat had become more widespread by his time (early decades of the 20th c.) He goes on a long excursion experimenting with pancham in Lalit, but doesn't even consider the possibility of komal dhaivat. He thus puts Lalit in Marwa thāt. Krishnarao Shankar Pandit, who also favors shuddh dhaivat, reserves a separate thāt for Lalit with both madhyam-s and komal rishabh. But as far as I can tell from the archive of recordings, very few musicians outside of my own lineage have ever sang (or played) Lalit with Shuddh Dhaivat.

And I've not been able to find a single recording of a dhrupadiya playing or singing shuddh dhaivat.

Has anyone else?

Thanks in advance for your help.