"You should prefer passing NULL instead of choosing your own defaults. "
I am using phoc completely standalone (no gnome, no phosh) and right now I don't see any way how I can pass keymaps. By relying on the xkbcommon fallback logic I can now set my keymap using an environment variable. i.e.:
XKB_DEFAULT_LAYOUT=se ./build/run -E alacritty works now.