The harmonium has disappeared entirely from the carnatic concert stage actually.
Mea culpa. As a trained Carnatic vocalist and violinist, I let my biases through. However...
> Harmony is great, it doesn’t mimic to do the vocal variations - just support the singer doing it as well as possible.
But the harmonium in Hindustani doesn't have harmony. I know enough Hindustani music to understand that only melodies are played, and almost never harmonies. Especially when the artiste is performing alap.
> use of Harmonium as an accompanying instrument is much more common across the spectrum (both north and south)
Like I said, it's far, far more common in Hindustani than in Carnatic. If some South Indian concert uses it, it's usually for bhajans, or, like you said, 'semi-classical' or 'light music'. The violin is de jure and de facto the default accompanying instrument for Carnatic music.
> I’ve never seen violin being used in Hindustani.
Kala Ramnath, V G Jog, N Rajam, M S Gopalakrishnan (although he was much more famous in Carnatic circles). The violin is not as common as the sitar in Hindustani music, but it definitely exists, and is increasing in popularity... Also owing to the reasons I mentioned.
> I know enough Hindustani music to understand that only melodies are played, and almost never harmonies. Especially when the artiste is performing alap.
I think is broadly true, but anecdotally speaking, myself and people I’ve trained under use harmony and love harmony for hardcore Hindustani classical. It’s a bit of a shame that more don’t use it, but I wouldn’t underestimate it either.
For context, I’m trained in Hindustani music and have given a number of performances and have family and friends who have done the same for Carnatic music. My experience is that Harmonium is very common as an accompanying/supporting instrument - probably precisely for the reasons you specified. Harmony is great, it doesn’t mimic to do the vocal variations - just support the singer doing it as well as possible.
What I’ve actually seen is that use of Harmonium as an accompanying instrument is much more common across the spectrum (both north and south) than Violin for classical and semi-classical - though unlike Violin it’s almost never used solo. OTOH I’ve never seen violin being used in Hindustani.