There is a similar destruction scenario for Ishk and Tagrum later on.
To save Tagrum, you need to visit the Elder after getting the key-hunting quest but before going to Engineer's Palace.
For Ishk, you need to get a Mana Heater Core from Claudia and charge it with one of the lightning spells before forging the Divine Key
Magus goes away forever if you don't beat her before beating Treu in Engineer's Palace.
I'm not 100% sure on whether there are missables concerning Dhistahn and the rematch in the mines, I've only played post-patch once.
There is a trainer in the Hallows that gives you multiple passives, but 3 of them require 1 of the game's three different limited armor upgrade items: Rogue's Cowl, Warrior's Plate and Magician's Cape. I think the passive requiring Rogue's Cowl is a double-jump.
The biggest missables though, are 5th weapons for each character. After beating the new superboss in Hall of The Fallen (near the mana crystal mines), you get a blacksmith with a weapon recipe that requires ALL TEN of the game's Tamahagane. If you want to unlock this weapon for current and following playthroughs, do not use Tamahagane on any weapon upgrades
Post edited May 18, 2015 by Nordicus