multiple monitors – Mirror screens, with main screen configured in portrait mode

I'm installing Ubuntu 18.10 step by step on my EZPad Mini Jumper 3, a cheap Chinese Windows tablet. I have already made it to the original usable state (WiFi / BT works) and now I'm testing the multi-displays features. In doing so, I have encountered a somewhat unique problem.

This tablet, like many other 8-inch Chinese, seems to have a display configured to mount in portrait (?) – the boot logo, BIOS GUI, GRUB, etc., is displayed in portrait. After booting into the OS, it also remains in portrait mode, until I activate autorotation, which allows the screen to rotate properly in landscape mode. However, the resolution of the screen shown in Settings is always in portrait mode (800×1280). Manually set a resolution in landscape mode (eg 1280×800), either via Settings or via xrandr, lock the tablet and turn on the black screen, forcing me to restart with the key combination REISUB.

xrandr exit under portrait:

Screen 0: minimum 320 x 200, current 800 x 1280, maximum 8192 x 8192
DSI-1 connected to primary 800x1280 + 0 + 0 (normal inverted left to right, y-axis), 0mm x 0mm
800x1280 60.01 * +
1280x800 59.99 59.97 59.81 59.91
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 56.25
840x525 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 59.98
800 x 450 59.95 59.82
640x512 60.02
720x450 59.89
700 x 450 59.96 59.88
640x480 60.00 59.94
720x405 59.51 58.99
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98
576x432 60.06
640x360 59.86 59.83 59.84 59.32
512x384 60.00
512x288 60.00 59.92
480 x 270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320 x 180 59.84 59.32
DP-1 disconnected (left axis reversed from the right x axis)
HDMI-1 disconnected (Inverted axis of the right y-axis reversed to the left)
DP-2 disconnected (axis reversed from right y axis)
Disconnected HDMI-2 (x-axis axis x axis), right and left normal)

… and under landscape:

Screen 0: minimum 320 x 200, current 1280 x 800, maximum 8192 x 8192
DSI-1 connected primary 1280x800 + 0 + 0 right (left normal inverted right axis x y axis) 0mm x 0mm
800x1280 60.01 * +
1280x800 59.99 59.97 59.81 59.91
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 56.25
840x525 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 59.98
800 x 450 59.95 59.82
640x512 60.02
720x450 59.89
700 x 450 59.96 59.88
640x480 60.00 59.94
720x405 59.51 58.99
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98
576x432 60.06
640x360 59.86 59.83 59.84 59.32
512x384 60.00
512x288 60.00 59.92
480 x 270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320 x 180 59.84 59.32
DP-1 disconnected (left axis reversed from the right x axis)
HDMI-1 disconnected (Inverted axis of the right y-axis reversed to the left)
DP-2 disconnected (axis reversed from right y axis)
Disconnected HDMI-2 (x-axis axis x axis), right and left normal)

When connecting an external monitor, the default mode is attached mode and autorotation is ignored, which resets the main display to portrait mode. In this case, I can follow the instructions given in this answer to manually apply a rotation to the main screen, which makes it usable.

Mirror mode is a different story though. The available resolutions are all in landscape and the choice of one of them will probably adjust this resolution on the main screen, which will freeze the tablet / screen.

Depending on the situation, is there a way to make mirror mode work properly? In addition (optional), can I configure Ubuntu to recognize the display as mounted in landscape?