LE:
FIXED! 
All I needed to do is to install xserver-xorg-input-evdev and xserver-xorg-input-mouse.
Original post:
Alright, so I did some more digging and found out that the displays are rendered using Tkinter on a canvas-basis system.
Initially I thought the UI is frozen because of a main-thread lock issue (looks to be very common with tkinter), but since I found out that the midi icon, the CPU load and so on are being displayed on a status canvas, this is out of the question. So tkinter is working.
But the issue looks like there’s no input to tkinter for either mouse pointer/buttons or keyboard.
I may be wrong but tkinter runs on top of X, thus X should be passing any mouse/keyboard events to tkinter.
So, my suspicion is that X does not properly handle the inputs.
libinput-bin and libinput10 are installed.
Are your mice mounted in any way under /dev?
Maybe X is looking for the mouse on some specific path which does not exist OR the mouse/keyboard are not automounted properly
Found these lines at the end of X log:
[ 2729.172] (II) config/udev: Adding input device r_gpio_keys (/dev/input/event0)
[ 2729.172] (II) No input driver specified, ignoring this device.
[ 2729.172] (II) This device may have been added with another device file.
[ 2729.179] (II) config/udev: Adding input device 2.4G Wireless Keyboard&Mouse (/dev/input/event1)
[ 2729.179] (II) No input driver specified, ignoring this device.
[ 2729.179] (II) This device may have been added with another device file.
[ 2729.183] (II) config/udev: Adding input device 2.4G Wireless Keyboard&Mouse (/dev/input/event2)
[ 2729.184] (II) No input driver specified, ignoring this device.
[ 2729.184] (II) This device may have been added with another device file.
[ 2729.188] (II) config/udev: Adding input device 2.4G Wireless Keyboard&Mouse (/dev/input/event3)
[ 2729.188] (II) No input driver specified, ignoring this device.
[ 2729.189] (II) This device may have been added with another device file.
[ 2729.193] (II) config/udev: Adding input device 2.4G Wireless Keyboard&Mouse Consumer Control (/dev/input/event4)
[ 2729.193] (II) No input driver specified, ignoring this device.
[ 2729.193] (II) This device may have been added with another device file.
[ 2729.197] (II) config/udev: Adding input device 2.4G Wireless Keyboard&Mouse System Control (/dev/input/event5)
[ 2729.197] (II) No input driver specified, ignoring this device.
[ 2729.197] (II) This device may have been added with another device file.
[ 2729.202] (II) config/udev: Adding input device GeneralPlus USB Audio Device (/dev/input/event6)
[ 2729.202] (II) No input driver specified, ignoring this device.
[ 2729.202] (II) This device may have been added with another device file.
[ 2729.206] (II) config/udev: Adding input device sunxi-ir (/dev/input/event7)
[ 2729.206] (II) No input driver specified, ignoring this device.
[ 2729.206] (II) This device may have been added with another device file.