Posted to Mastodon about the issue I was having with the header I wanted to use on the blog home for this site–and actually got a solution, yay!
The way to solve the problem turned out to be to adjust a setting on the Cover block at the top of the page. Instead of telling it to try to use a Featured Image, I just told it to use the specific image I wanted out of the Media Library to begin with.
So many thanks to django on Mastodon for pointing me in the right direction! And for helping me identify a nice easy solution that did not require me to go digging into the underlying PHP of this theme.
(Which I could do, I just don’t wanna. It’ll be a more productive use of my time to just work on building this site out so I can make it go live.)
On to adjusting the layout on the Cover blocks I’ll be putting on all the pages, for general consistency of style! And at least now I also have an assortment of cover images to choose from when I’m writing posts.