> You could try to serve the Memento from the server and store it in an index db

I am not sure what you mean by "store it in an index db", but I was thinking about using the searcher on a static website (no real backend, only a fileserver serving pre-generated html files). So if I understand you correctly, in order for this to work I would have to cache Memento via a local storage and load it on every page load/search request.

Unfortunately the index would change over time, thus one would have to detect this somehow and regenerate Memento as well.