Well in your case, as all your tuners are on the core, then you should see them in tv_shows_1. Your recordings should be available already to every media director. You should not need to force anything. Mine vary by moon, because all my MD's have tuners. If they were all to reside in the same machine, the all recordings should go to tv_shows_1. And that folder you can mount to any other drive/directory structure, just make sure you have the path set to the new location in mythtv-setup.
I have 4 stb's and they all have a LiveTV<stbname> entry, and they are all visible to every MD. It would be worth looking into the mythtv recording settings, to see if you can assign a recording to a particular MD, and that would turn point the recording to a different directory. I have not tested this though, so it would merely be a guess.
As to the need to force recordings to a specific directory, if you have all your tuners in one box, that should not be necessary. All MD's capable of playing video/mythtv will be able to see them, either through the provided "Video" button in the LMCE menu, or by choosing recordings from the LMCE menu while viewing TV (mythtv). This is how I prefer to do it, as I commflag all my recordings, and watching them back through the mythtv interface, in my case, allows for automgic commercial skipping (you have to enable this on each mythtv frontend, via the setup options inside mythtv) and it is not as cpu intensive as using the xine_player to view the recordings.
And pointing the mythtv-setups to that mounted large drive, is basically exactly what I do, so it should work fine.