ToME: the Tales of Maj'Eyal

Stone Warden's damage types are all over the place -- Physical, Arcane, Nature, and a touch of Fire. The class's only inherent source of resistance penetration is Crystalline Focus for physical, and unless you're going for a heavily caster-focused build it really seems like that's the primary thing you'd be getting for investing the category point in Stone.

Now that Tarrasca's gone and blocking is a viable augmentation to a Stone Warden's defenses, is Plate of the Blackened Mind essentially mandatory for this class to play on Insane? (Assuming PotBM's Dominate checks the user's mindpower, I expect it's much less useful on Madness given the high mindsaves there.) Is there another reasonable option for Stone Wardens to get a solid amount of resistance penetration in more than one element?

Well, it depends on what you consider reasonable, but if you invest heavily in it Deeprock form gives 24% ish damage bonus and 15% ish respen in your three main elements. Drem can snag pretty good uptime on it - 15/25 if only using off Frenzy with max Frenzy investement, 16/23 if max Frenzy with Adept, or initially 30/45 without additional Frenzy investment or Adept and reapplying as soon as it drops.

Would temporal form viable? The uptime isn't great, but it also gives you an anomaly that reduces resistances for 8 turns

I dont know why you're talking about armor selection under this topic. I mean even if you're wearing robes you're still only getting physical penetration from egos (though a lot of it).
Nature and arcane penetrations are very rare on egos. Nature penetration only appears on a single mindstar ego, and arcane a single cloak ego. So I guess you just use normal attacks against anything with high resists.

Tarrasca used to be automatic best-in-slot for Stone Wardens due to the Earthen Vines tree slowing your movement by 50% and Tarrasca giving you resist all = any slowed movement factor. Now that Earthen Vines no longer slows the armor slot can consider other contenders.