Wow.

After getting rid of the GDK_SCALE stuff (well, replacing it by querying
GTK for what scaling it's using), and doing the same with the
positioning as Ryan did (I ended up writing my own version to make it
fit better with the call conventions of the old GDK_SCALE stuff)...

Things just work.

That is, my Emacs opens with the right size window, unmaximized, and
everything is hunky dory.  I think.

Let's see...

Here's "emacs -Q":