Fair point, although in the first case the modeline gets healed after:

    C-h i

    C-x k RET

so emacs apparently knows how to change the default medium font to bold under some circumstances.  Perhaps the bold font is being instantiated prior to the default font getting re-instantiated based on my command-line definition.  It seems to me that changing the default font on the command line should change all the derivative fonts.

Anyway, I tried to use customize-face to set the default font, but I'm having some difficulty.

Starting again from "emacs -Q":

After I invoke M-x customize-face default, the customize buffer says the Font Family is courier and the Font Foundry is adobe.  I click in the courier box and replace "courier" with "dejavu sans mono" (w/o the quotes).  I click in the adobe box and replace "adobe" with "misc".  Next I held State button to set the font for the current session and instead of changing to "dejavu sans mono" the font changes to some kind of Times font.  Showing the lisp expression confirms this: