I did not read the content of your post - and the answer is YES!

Name one purpose other than for the sake of it. That makes it fine art.

I heard good things about emclient many times, but it’s not for Linux/BSD,etc…

Really never had any problems with that. The only mail app I know that made problems indexing mails or keeping the index intact was Microslop Outlook for Mac. Anyways, lean back and search for the fault outside of the software scope. I guess you have either hardware issues or system configuration issues. With any mail app it should also be possible to reindex all mails. If your app stores mails as files you can also just grep the Mail folder and crossreference the E-mail’s unique ID and then access it through your app. Also fairmail’s search feature always worked flawlessly for me.