Thanks for your handling of this!
My 2¢:

• Putting it at the very top sounds fine and easy. (Leave it to perfectionists to move if they don't want it there.) (If being perfectionist, I'd skip all templates at the top and then insert. A perfectionist implementation could do that.) Having a parameter seems baroque.
• {{archive box auto}} can support a variety of schema. Better yet, I think {{:Special:PrefixIndex/{{PAGENAME}}/Archives/}} should work on a wide variety of subpage schema?
• Does your "no incoming link from the original page" test not handle this difficulty?

Thanks for creating this.
Thinking about strategies.
Assertions:
Every language/wiki should have an archive box template.
One more variable pointing to a language-specific template ({{archive box auto}} for each language) is trivial compared to the alternative of not having this feature and maintaining links to archives manually. (And perhaps the variable already exists?)
I can't think of a case where we would want to allow auto archiving if the archive page to be created would have no incoming link from the original page. So I think having an option/setting is inappropriate.

Tried it. Disabling http2 as @BBlack /@Thibaut120094 suggested eliminates the error for me too.
It seems to occur only if the browser's connection to the site has been idle for a few minutes. So it's probably timeout related. (Tested at User:Elvey/sandbox on en but didn't nail it down.)

Seeing this error too. For me too, "it occurs when saving almost any edit", (or at least most edits) since a couple days ago.

Hmm. Now, a few hours later, the page looks fine. I guess it just takes a while (half hour? 2 hours? what?) for a bot somewhere to do it's job? Can anyone explain? Reclose?

I'm not seeing this as fixed. I just renamed https://commons.wikimedia.org/w/index.php?title=File:497bg-869sq-42-24592-dauntless-dolly.jpg&redirect=no

This doesn't show up on the new page:

File usage on other wikis

FYI, the biggest advantage of OLED displays, which are becoming steadily more prevalent (Samsung phones and tablets, LG phones, Apple Watch, LG's 55EC9300 TV/monitor, etc) is the potential for less energy use and longer battery life, particularly with a dark user interface. Our site is so popular that simply popularizing light on dark would save significant energy.

This is relied on by the hidetopcontrib javascript that many thousands of editors seem to use, per search

Summary: I'd say that it 'should just work' if someone is using the "Use a black background with green text on the Monobook skin" gadget, so I'd suggest making that mode work well with Flow would make testing easier and perhaps make more sense/help many users, including me. But if you want to know what I do and why, keep reading...