hplip 3.16.2: no installed HP device found (while CUPS finds it)

I was investigating further an issue already reported here. The problem is: after having upgraded the hplip driver to 3.16.2, the scanner in my all-in-one printer HP Color LaserJet Pro MFP M277dw does not work any longer (while the printer does). Today I found other oddities that seem specific to hplip rather than to sane, whereby this other post.

I use Ubuntu Linux 14.04 LTS. In all that follows the device is connected and powered-on. The hplip page for that device is here.

Evidences

A. Moving on from the sane community page on Ubuntu, I followed the suggestion to run sudo hp-setup. The answer is

warning: CUPSEXT could not be loaded. Please check HPLIP installation.

B. If run hp-doctor, the welcome message is

error: This distro (i.e ubuntu 14.04) is either deprecated or not yet supported.

This sounds utterly odd to me, because the previous hplip did not dare to complain this far of the very same distro. The complete output of hp-doctor is available from here on Paste Ubuntu.

C. Ever more puzzling, if I open the HP device manager, I am presented with the window

i.sstatic.net/kh3zt.png

which seems a false statement to me, since the device works as a printer at the very least. If I click on Setup device... I get again the same dialogue window. And CUPS on localhost:631 indeed confirms that the printer is there ready to be found, nice and idle.

Questions

Is there a way to have the commands hp-setup and hp-doctor run smoothly so that I can fix the scanner issue down the line?

If not, how do I downgrade the hplip driver to the previous stable version? Installing 3.16.2 has led to more havoc than joy.