One problem I see with Tels idea for Shadows and a dead magic plane ... their special abilities are (Su) and hence won't work for them in a dead magic area anymore than the parties items, spells, etc., so no Strength Drain, no spawn creation. It'll be a standoff at best not Shadows squash APL 30 (or whatever) party. Otherwise definitely ups the difficulty of getting to the inner most demi-plane fortress.
Can't normally link or use Mind Blank as a companion to Hallow ... though given that's a list of Mythic beyond 20th type stuff I'm not sure how pertinent that is. You can normally link Dimensional Anchor to Hallow however.
Or rather than go with Planar trait Dead Magic go with Sentient plus Morphic to the will of the plane itself.
Sequester is a very potent 'lets hide this' spell (from either remote or direct observation). One could debate whether even Discern Location defeats Sequester and even if you say yes you still need to have physically touched the object previously for Discern Object to work. Build a front door over your portal to the plane letting no one else see or touch it ... then hit it with Sequester. Bonus points for your Sentient Plane keeping access to the door/portal always walled away for "Maaasster".