[search indexed files that are offline] One would hope this is not possible.
I think the idea is store the search index in a separate place from the file. For indexing text though, I’ve found that the index is comparable in size to the file itself. It’s not entirely clear to me what OP wants to search. Something like email? Obviously if it’s just metadata for media files (kilobyte text description of a gigabyte video) then the search index can be tiny.
Real-time updates as files change
Would require non-portable script that stores each file’s mtime in an array and compares the old mtime against the new mtime using stat, and then loop. Maybe implement as a daemon.
That is what inotify is for.
I realize your overall answer was mostly snark, but the problems mentioned really do take some work to solve. For example, if you want to index email, you want the indexer to understand email headers so it can do the right things with the timestamps and other fields. You can’t just chuck everything into a big generic search engine and press “blend”.
I will mention git-annex which is for sort of a different problem, but it can help you manually track where your offline files are, more or less.