After some thinking, I found a very simple solution to the RTL issue.
Instead of trying to force SilverBullet to support RTL through a plug—which turned out to be too restricted—I converted my original JavaScript code into a Firefox browser extension.
I named the extension RTL Silverbulle, and it solved the Arabic text direction problem completely. Now SilverBullet works perfectly with Arabic, and the editor automatically switches between RTL and LTR just like I wanted.
The extension can also work automatically. In the Firefox extension preferences, you can add any website URL where you want the RTL fix to be applied automatically.
The extension is available on the Firefox Add-ons store and also on GitHub.
Thanks again for the guidance!