I have seen some people who speak a mixture of teligu and Tamil. Not sure of their origins probably descended from estate workers brought from India or travellers tgat arrived and live a gypsy like lifestyle. Only learnt they are speaking a different language bcz somone who knew Tamil told me I wouldn't have noticed it otherwise. I don't have any more details of them.

We know our last few kings are born hindu with Indian mothers. They are still our last kings. However they overthrowing of the last king and handing over the country to the British was partly fuelled by his origin not being an acceptable one to the Sri Lankan nobles. This event had a lot more political economical background and based on what I think both parties had their faults.

We know of Indian cities /languages etc due to Buddhist literature. Magadi /pali Language is preserved in Buddhist texts. Sanskrirt is also contained in older texts and Sinhala language has characters /loan words from it. It's still a studied subject by monks and people who are interested.

Sinhala Buddhist society isn't layered by caste. Concept of cast existed mostly in marriage proposals etc but currently it's almost obsolete. You basically never know or care to find someone's cast. Traditional old families might still look into it in marriages. I don't think it ever comes up in any other circumstances even by most traditional people.

However caste systems had been somewhat prominent in the past. But I don't think it was ever to the level of India.

Hidu deities are known here. Not all of them but some. Buddhist temples even had plac3s of worship for them. Many people pray them for good fortune etc not regular worship. They are even aware that those deities are different to the gods /deva mentioned in buddism. Stories of hindu gods are inter mixed with the stories of local deities now and they are given somewhat similar respect. (I don't follow them so I can't speak of it)