1. The CSS you added will really not shrink your text.
If you need your font to scale or shrink, you will need to follow the advice given in this thread.
2. In Customize > WooCommerce, try increasing the Thumbnail Width. For more details, please see https://theme.co/apex/forum/t/resizing-woocommerce-images/1706
3. Please ask the plugin author if the page can be displayed through a shortcode. If not, you will need to ask them what template they are using. I can the body class is using the blog class but not sure how your plugin works.
For 4 and 5, please contact the third party plugin's support for this because they are the ones who know the inner workings of the plugin. Also, we do not have support for third party plugins.
One tip I can give for number 5 is, you can get any element's selector by following this guide.