I haven't had any real issues getting Myth up. I'm definitely not using it to its full extent right now, but getting it to playback live tv wasn't an issue at all. I just went through the Myth Setup, then launched TV.
Here's the thing though... since you've set up your Pluto box as Core only, you have no Media Director (MD) functionality there. And since you aren't using Pluto's DHCP server, you can't set up a diskless MD. This means that at this point in time you're probably unable to play back media of any sort, which includes MythTV.
Being pretty new to Pluto myself and a long-time Windows user, I'd recommend that you rebuild your Core as a Core/Hybrid. Once you're familiar with Pluto and everything it does, then maybe rebuild it back as a Core. It'll be extra work in the long run, but it should also prevent you from banging your head against a wall. Hell, even as a Hybrid it still retains all the Core functionality, so if you never have a monitor connected to it you should still be ok for the most part (though I have noticed that some PnP alerts only occur on the screen of my Hybrid and not on other Orbiters, but that should only matter when adding new devices).