My Feedback was marked as "Investigation complete - Unable to diagnose with current information"

It was probably a mistake to mention this in a place where Apple Safari extension engineers can see it, because they might just quash your technique. ;-)

Sorry, I didn't look at your demo video before. I think I can reproduce your results now. The redirect works the first time I search from the address bar (although the web page crashes in Safari 17.6), but it doesn't work the second time, after a page is already loaded in the tab. Note that if you change the focus away from the address bar, you can see it's stuck in the middle of page loading progress.

The workaround is in the sample Xcode project.

It's actually a Safari app extension, so there's no manifest.json file.

Hmm, I'm still using V2. I wonder if that makes a difference.

As I said a month ago, it's too late. I already made the difficult decision to discontinue my app and remove it from the Mac App Store. I'm not going to reverse the decision now, especially since it's clear that this feature has a very low priority within Apple and may be broken without care or comment.