Ok, there seem to be a few things here. In the first case you are wanting to make a direct edit of books. I have made the plugin window modal deliberately, because if you were to make changes in the calibre window it will make the results displayed in the plugin window out of date. The primary use case I had in mind was where you would be perfectly happy renaming the authors via the plugin window. However in your case you had found a completely kakked metadata situation, and renaming the "A.N. Author" was not your desired action. I don't see that modality changing to be honest, I think it is an extreme situation and would cause too many headaches if I were to change it.
However your next point about that was that as you clicked cancel you found the plugin cleared the search so you could no longer see those rows to edit them outside the plugin. The intention on my part to restore whatever search the user had displayed before they ran the plugin, to ensure that data is refreshed visually. However I can see that in your situation you would instead want those results to be kept on screen. That is something I will take a look at changing - if a suer has "Show books on screen" checked, there is no issue with leaving them on screen when Close is clicked.
Your final KeyError message looks a concern as that looks a genuine bug. It looks like somehow the plugin was dealing with stale data. However to fix it I am going to need to be able to replicate it - if you or someone else comes across it again and can give me some steps it would be much appreciated.