Dear Prof. Pandurangi,
Regarding comprehensive study material for the
Ṛgveda, and in particular your question as to whether a
Ṛgveda reverse word index had been prepared by anyone: I do not know of a reverse word index. You no doubt already use the complete
Ṛgveda
word index prepared by Vishva Bandhu and his team. Besides this, there is another helpful study tool, A
Ṛgvedic Word Concordance, by Alexander Lubotsky, 2 volumes, 1997. It lists every word found in the
Ṛgveda, followed by the number of its occurrences there, and then it gives every pāda in which the word occurs. So you can easily see the usage of each word throughout the whole
Ṛgveda
in one convenient place. Of course, you often have to go to the whole verse, or even the whole sūkta, to get the context. But it provides a good start. I do not know if it is available online.
Best regards,
David Reigle
Colorado, U.S.A.