Greetings,
Thank you for reaching out to us. I hope you are safe and well!
1. The Settings.xml file is used only during the installation of the plugin, to populate the default values, not when you restart the application. So I wouldn't advise you to configure the plugin this way.
2. Yes, the settings starting with SaleCampaignProductPageCustomization are for the Product Page.
3. Yes, the settings starting with SaleCampaignCategoryPageCustomization are for the Product Box.
4. The Settings.xml file is only for the default values for the settings of the plugin. I would advise you to put the custom CSS under the Custom Head Styles section of the settings of our theme.
5. You can add custom CSS for the Product Box under the Custom Head Styles section of the settings of our theme.
Yes, it will increase the size on desktop screens as well.
If you want to reduce the font size on mobiles only, you would have to use custom CSS instead.