I got a comment to the effect that I was wrong and that the settings are available in the dashboard. After an inspection of the dashboard for this blog, and of every single customization tab, I conclude that I was not wrong.
You can only get compose-gui support for mixing RTL and LTR in posts if your blog language is set to Arabic, Hebrew or Persian. The relevant blogger help entry confirms this view (emphasis mine):
If you're not seeing the directionality buttons in the post editor, it's likely because right-to-left support is only available in the Hebrew, Arabic and Persian interfaces.I stand by my original complaints.