WetGeek
My reply gave me another idea -- the VBox driver was set to the default VMSVGA. I changed that to VBoxSVGA and its available resolutions included 1920x1080.
My apologies to Bodhi!
However, I still couldn't get the VM to expand to that resolution using Host+F, so I tried installing the guest additions .ISO, with this result:

Still unable to get anywhere. There may still be a way to make it work, but in my experience Bodhi is the hardest, most complicated distro I've ever tried to set up. Since I explore other distros strictly for fun, and this hasn't been fun at all, I'm willing to stipulate that this has been all my fault. But I'm still done with Bodhi.