I tried first your method because is looks simpler than the emulator. But all I get is a black wlroots window. Maybe you can understand what is going wrong with the following log lines. I am on ArchLinux. If complains about X0, then it complains about non-existent undefined.yaml BUT then it loads us.yaml. But nothing appears on the black wlroots window.
[atsol@thinkpad build]$ LC_ALL=C phoc -l 3 -E /home/atsol/tmp/build/src/squeekboard
(process:11326): phoc-wlroots-CRITICAL **: 10:33:54.803: [xwayland/sockets.c:63] Failed to bind socket @/tmp/.X11-unix/X0: Address already in use
Failed to load layout from Path: “/home/atsol/.local/share/squeekboard/keyboards/undefined.yaml”: Bad data: IO: No such file or directory (os error 2), using fallback
Key preferences has no keysyms
Loaded layout from Path: “/home/atsol/.local/share/squeekboard/keyboards/us.yaml”
libEGL warning: FIXME: egl/x11 doesn’t support front buffer rendering.
^C
[atsol@thinkpad build]$