Well, I finally made some tests. I don’t know if this makes any sense, but what I did was to run xrandr -q on all the configurations above (is it actually expected to produce any useful output on Wayland? 
). To my surprise, when I ran it on Wayland with the Nvidia proprietary driver, it did seem to detect the external monitor, even though it apparently failed to activate it:
Screen 0: minimum 16 x 16, current 3840 x 1080, maximum 32767 x 32767
XWAYLAND0 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 480mm x 270mm
1920x1080 59.96*+
1440x1080 59.99
1400x1050 59.98
1280x1024 59.89
1280x960 59.94
1152x864 59.96
1024x768 59.92
800x600 59.86
640x480 59.38
320x240 59.52
1680x1050 59.95
1440x900 59.89
1280x800 59.81
720x480 59.71
640x400 59.95
320x200 58.96
1600x900 59.95
1368x768 59.88
1280x720 59.86
1024x576 59.90
864x486 59.92
720x400 59.55
640x350 59.77
XWAYLAND1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 340mm x 190mm
1920x1080 144.04*+
1440x1080 144.01
1400x1050 144.00
1280x1024 144.05
1280x960 144.13
1152x864 144.09
1024x768 143.87
800x600 143.83
640x480 143.85
320x240 142.05
1680x1050 144.07
1440x900 143.99
1280x800 144.00
720x480 143.85
640x400 144.04
320x200 141.40
1600x900 144.04
1368x768 143.93
1280x720 143.85
1024x576 143.91
864x486 143.63
720x400 143.88
640x350 143.57
The XWAYLAND1 primary is indeed the primary display. The XWAYLAND0 is the external monitor, which seems to be positioned to the right of the primary one.
So, based on the output above, do you guys believe there is any chance of using the external monitor with Wayland?
Just as a baseline, here’s the output of xrandr -q with Nvidia + X11 (the external monitor is positioned above the primary one):
Screen 0: minimum 320 x 200, current 1920 x 2160, maximum 16384 x 16384
eDP connected primary 1920x1080+0+1080 (normal left inverted right x axis y axis) 344mm x 193mm
1920x1080 144.15*+ 60.20
1680x1050 144.15
1280x1024 144.15
1440x900 144.15
1280x800 144.15
1280x720 144.15
1024x768 144.15
800x600 144.15
640x480 144.15
HDMI-1-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 477mm x 268mm
1920x1080 60.00*+ 59.94 50.00
1680x1050 59.95
1280x1024 60.02
1280x960 60.00
1280x720 60.00 59.94 50.00
1152x864 60.00
1024x768 60.00
800x600 60.32
720x576 50.00
720x480 59.94
640x480 59.94 59.93
Anyway, the format seems intuitive enough, so that shouldn’t be much of a problem.