I sympathise, I don’t think this endgame can easily be solved without discovering the author from the lost letters. So in that sense it’s a real test.
I’ve always found indices in dictionaries of quotations problematic. Take Jacques’ “All the world’s a stage” monologue in As You Like It. I can think of 30 words which could easily be justified in an index, but it ends up being nearly the same length as the list of quotations, as each has a line to itself.