In that case I agree that the Bylines plugin is the best option.
I was thinking about your issue, and if you don't need the author names to be linked inside WordPress (that would mean that the visitor could click on the name and would see all related posts), then it is fairly easy to add a custom field to the posts where you could fill in the name(s) right in the post editing panel, and with just a little bit of coding in the theme, it would show the name(s) you have typed in. I am not trying to convince you that you don't need the Bylines plugin or that it is not a good solution, it is just an additional option for you. Of course, I can provide you all the code necessary if you would like it this way.
By the way, I would like to mention, just to be sure, if you did some modifications to your theme's files, you have to create a child theme in order to preserve the modifications you have made, because every change in the theme files gets deleted when WordPress makes an update (these days it happens automatically).
If you have any questions, just feel free to ask, and I am really happy to assist you.