This is the current $XDG_RUNTIME_DIR situation, running mir-kiosk and my snap in service mode directly on a 18.04 host with edge core.
➜ sudo tree /run/user/0
/run/user/0
├── snap.wpe-webkit-mir-kiosk
│ └── wayland-0 -> /run/user/0/wayland-0
├── wayland-0
└── wayland-0.lock
➜ sudo ls -laR /run/user/0
/run/user/0:
total 0
drwxr-xr-x 3 root root 100 Mär 7 16:34 .
drwxr-xr-x 5 root root 100 Mär 7 09:47 ..
drwx------ 2 root root 60 Mär 7 16:34 snap.wpe-webkit-mir-kiosk
srw-rw-rw- 1 root root 0 Mär 7 16:34 wayland-0
-rw-r----- 1 root root 0 Mär 7 16:34 wayland-0.lock
/run/user/0/snap.wpe-webkit-mir-kiosk:
total 0
drwx------ 2 root root 60 Mär 7 16:34 .
drwxr-xr-x 3 root root 100 Mär 7 16:34 ..
lrwxrwxrwx 1 root root 21 Mär 7 16:34 wayland-0 -> /run/user/0/wayland-0
user@host:~$ snap version
snap 2.38~pre1+git1182.68177c3~ubuntu16.04.1
snapd 2.38~pre1+git1182.68177c3~ubuntu16.04.1
series 16
ubuntu 18.04
kernel 4.15.0-46-generic
And for reference, this is still the only traceable error (highlight by me):
user@host:~$ snap logs wpe-webkit-mir-kiosk
2019-03-07T15:43:59Z systemd[1]: Started Service for snap application wpe-webkit-mir-kiosk.browser.
# ----- ⬇︎
2019-03-07T15:44:00Z -[5235]: Failed to create WPE view backend
# ----- ⬆︎
2019-03-07T15:44:00Z systemd[1]: snap.wpe-webkit-mir-kiosk.browser.service: Main process exited, code=exited, status=1/FAILURE
2019-03-07T15:44:00Z systemd[1]: snap.wpe-webkit-mir-kiosk.browser.service: Failed with result 'exit-code'.
2019-03-07T15:44:00Z systemd[1]: snap.wpe-webkit-mir-kiosk.browser.service: Service hold-off time over, scheduling restart.
2019-03-07T15:44:00Z systemd[1]: snap.wpe-webkit-mir-kiosk.browser.service: Scheduled restart job, restart counter is at 5.
2019-03-07T15:44:00Z systemd[1]: Stopped Service for snap application wpe-webkit-mir-kiosk.browser.
2019-03-07T15:44:00Z systemd[1]: snap.wpe-webkit-mir-kiosk.browser.service: Start request repeated too quickly.
2019-03-07T15:44:00Z systemd[1]: snap.wpe-webkit-mir-kiosk.browser.service: Failed with result 'exit-code'.
2019-03-07T15:44:00Z systemd[1]: Failed to start Service for snap application wpe-webkit-mir-kiosk.browser.