If setting up a core, can one safely skip all tv/audio setup, boot into Linux MCE, switch the desktop and and check only core to start?
Yes, it's safe to do that.
You can't use the Orbiter User Interface from a monitor connected to the Core if you do that, but you can still administer the Core (from the Core's monitor) using the LinuxMCE Admin Website (which needs no special graphics or sound). That generally is preferable anyway.
There are caveats -- if you output from the Core's soundcard directly to a whole house audio system, you should setup audio.