You can change your Plasma session from X11 to Wayland by selecting the session type at the login screen (Wayland is the default for Plasma).
It has been a while since I used a live USB, but I am fairly sure that you can still log out of a live Plasma session, although I am not sure what the default user password is for the live image. Edit: I just did a quick search - the user & password should both be manjaro
If you have set up autologin to skip the password screen, you can change to a Wayland session by opening System Settings → Colors & Themes → Login Screen (SDDM) → Behavior.