Never seen this done before so it will be interesting to know if this works out for you…but your question number 1 is one I cannot answer because I’ve never seen this done before. As for the relevant
and what file, this would be the “sidebar-showcase.php” file. However, please be aware that if you modify the theme files and you update the theme, you will lose the changes you made, which is why a child theme is best for making these kinds of modifications to the parent theme (the actual theme itself). Celestial Reloaded does come with a child theme you can install, but also be aware if and when you decide to use it, upon activation, WordPress will see it as a new theme activation. This means you will need to redo your theme option settings, re-assign widgets, your menu, etc. Unfortunately, this is just how WordPress works.
As a side note: When you paste code into the forum posting, please highlight the code and then on the editor you will find a icon button that looks like this and when mousing over, it says “Code”. I edited your post for you to do this, so now your code is wrapped in that grey box. If the code is displayed as you had it, it could conflict with the actual page coding for the forum because the browser might think it’s actually page code B)