Hi,
1. That text was already there, it informs you about using the custom ajax handler. It was activated all along. You can completely ignore that message.
2. I did not change any styling, any custom code nothing. I only made a change in the core search files, which are completely unrelated to the actual search itself. Plus I had to save the option like I did before, nothing else was changed. I suspect this is a cache related issue, CDN or minify caches tend to have a long reload time. I have changed the following options to try bypassing that: https://i.imgur.com/1OZ4bsM.png
These are the default options, I recommend leaving them as they are. The change should be apparent on the front-end now.
3. You may have to ask the plugin author about that. I only changed a single line on the core query files, but nothing about display or any other files. I didn’t even notice that plugin until now.