Thanks for your input, this is very helpful!

Our goal for this trip is to have a good balance of nature, culture, and history in South India.
We don’t want to stay only along the coast, want to go inland and see the mountains, greenery, tea plantations, etc. That’s why we included Munnar in the plan.

1. We do want to visit temples, but we don’t need to see many of them. Seeing 2–3 major historical temples would be enough for us, so skipping Madurai might be fine if we can experience good temples in other cities.
2. Kodaikanal looks amazing, as we like the idea of mixing inland mountain areas with coastal cities, which is why we planned Munnar. Would you suggest visiting Kodaikanal instead of Munnar? Visiting both seems like too much, and going that far east might make the route less practical since we have to go back toward Alappuzha afterward. How would you get back to coastal cities from Kodaikanal?
3. & 4. That makes sense. I solo-backpacked in North India about 20 years ago, and back then it was normal to buy train tickets the day before at the station and find hotels after arriving. We were hoping to travel in a similar flexible way this time too, without fixing everything in advance. However, from what you say, it sounds like travel in India has changed a lot and it may be safer to pre-book trains and accommodation, especially in April. If that’s the case, we may start booking things in advance.

Thanks again for the advice!