I see that X Card has the ability to set a custom image for a share card on X. Is this setting the og:image meta data in the header? Do BlueSky and Mastodon use that same meta data for deciding which image to show in the share card when I post a URL link to a blog post?
Can the X fields be relied upon to set display information for sites other than X? I need these to show a unique image that is similar to, but different than, my site’s header image.
Should I instead add custom HTML to the site to provide the og:image of my choice? Will that interfere with the normal display of the site’s header image in situations like the newsletter or mobile display?
Setting the Facebook image will populate the og:image field. If it isn’t set, having a featured image will populate the field instead.
If you’re content with your feature image being the preview image, it’s not necessary to set the facebook and x settings manually, but if you want more control, you have that option.
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Do we know if setting the Facebook image propagates to BlueSky?
This post suggests that the og: prefixes will work, but is this true? I mean, I can just try it on my next post, but having some assurance rather than wasting my efforts would be nice.
Try it out and report back! There’s no secret black box here. Either you’ll see it being set (look at the source code via ctrl-U) or not, and either you’ll see Bsky pick it up or not. 
Well, it’s hard to test.
I swapped the blog SITE banner and I made an og:image graphic that contains branding.
This specific blog post was posted previously onto Bsky and Mastodon.
Reposting the same link results in the OLD site banner being redisplayed, presumably cached in Bsky and Mastodon somewhere. Posting into Discord results in the new og:image as I had hoped, so I think this is going to work for future posts? 
But I can’t verify.
It is a little frustrating to have to do these experiments on the live blog, with real posts to my readers. If it doesn’t work next time, then because of the caching I won’t have another chance to fix it until the NEXT blog and so on. It’s not end-of-the-world stuff here, but at the very least having some official support for more than just X and Facebook would be keen. 
I can say that the magic sauce is in the tags of your post. As you suspect og:image is the one that is most often picked up as a share image (what you see in the unfurled link in other social media)
OpenGraph (OG) was originally developed by Facebook, but is now widely adopted across most sites that do link unfurling, with the exception of X (fka Twitter) - hence the separate twitter data. You can read more here. https://ogp.me/ - even then, many sites will look for og or twitter data first, then fall back to standard meta content tags.
You’re right in that most social networks will aggressively cache OG data, and AFAIK, there’s no way to force any kind of refresh from your end.
One thing to be aware of - Not all link unfurling code displays images the same way. So, whilst FB might display a large ‘card’ style link with a 800x600 image, some might have different layouts and resolutions. There’s quite the debate online about the best size - but I found this article to be quite helpful - Social Image Size Specs: One OG Image to Rule Them All!
Hope this helps
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og:image has indeed solved my problem on that matter. So far! 