Hi,
1. You can actually turn that off here. It was added because some users wanted to make redirections from empty tag permalinks to different pages. It was requested many times, so we added it as a feature.
The archive page breadcrumb and layout is controlled by the theme. All the plugin does in this case is display the “marci” tag and the link to that archive page, which is requested from WordPress core. There is probably a way to change that text somewhere in the theme settings, usually most modern themes have a customizer feature where you can change the archive breadcrumbs and the text as well.
2. I just logged in to check, it seems to be working all right on my end. I only see the tagged posts in the results, for example: https://i.imgur.com/mehGTNV.png
