Headphone Buttons and USB-C Adapters
Using the keytest app, I found out that the device ID of the headphones     (2) differed from the device ID of the regular volume buttons (7 and 3).
Now, I only had to find out the device name corresponding to the device ID. This I did using a terminal emulator:

$ su
$ getevent
add device 1: /dev/input/event7
name: "msm8976-skun-snd-card Headset Jack"
add device 2: /dev/input/event6
name: "msm8976-skun-snd-card Button Jack"
add device 3: /dev/input/event4
name: "qpnp_pon"
add device 4: /dev/input/event3
name: "qwerty"
could not get driver version for /dev/input/mouse1, Not a typewriter
add device 5: /dev/input/event2
name: "hbtp_vm"
add device 6: /dev/input/event1
name: "input_mt_wrapper"
could not get driver version for /dev/input/mice, Not a typewriter
add device 7: /dev/input/event5
name: "gpio-keys"
could not get driver version for /dev/input/mouse0, Not a typewriter
add device 8: /dev/input/event0
name: "synaptics_dsx_s2"


According to the documentation, all characters other than 0-9, a-z, A-Z and - are replaced by _. Thus, I created a new file: /system/usr/keylayout/msm8976-skun-snd-card_Button_Jack.kl with the following content:

# Configuration file for LeEco Le 2 headphone buttons
key 114 VOLUME_UP
key 226 HEADSETHOOK
key 259 VOLUME_DOWN


After a reboot, I got the expected behaviour!