I like the idea for defense focus mobs, but I feel like their great rewards should be balanced, and what exactly are their Great Rewards? I'm currently thinking diamond shards would be a cool addition to the game that could be a drop to them with a slightly lower drop rate than that of Gold Nuggets from Zombie Pigmen, and function the same way. I also think, due to the way ice works in your world idea, that if they teleported after they lose too much health, travelling to them to get them would create more work for the potential reward! I really do like the extra challenge of absolute ice, it reminds me of gym and enemy hideout puzzles from the Pokemon games, which I loved solving--plus outside of the Fold, it would be a really cool mechanic for all sorts of minigames and other stuff. I do feel as though Absolute Ice is cool enough for simply its movement mechanics. I don't feel as though turning all nearby lava and extinguishing nearby flames would be interesting, however, and a bit overpowered for a block. Being immune to flames and explosion is good, however.
Overall, I really like this suggestion, and I would also hope there would be a lot of super rare but extremely rewarding structures for one thing: it's Hypospatial. You could make a portal in the Overworld, explore around one area, come back, then make another portal a few hundred blocks away in the Overworld and see an entirely new section, and I would LOVE that. Minecraft has been wanting to encourage exploration, and a world like this would be perfect: rewarding exploration with an easy way to access new areas. Where the Nether feels claustrophobic, I would love to see the Fold be very open, with less height, more of an arctic sprawl with big icycles hanging down from a roof you can't see from the ground due to a heavy white.
Also, in one last note, I don't think all mobs should be defense based: there should be, scattered among the high reward, high defense mobs, be protector mobs, with about the difficulty of Endermen in their health and attack power, that don't have as good of drops and become hostile alone, or with a slight compromise to their health and attack power if this concept is used instead, in groups, when a defending mob is attacked.
Hope this helps you brainstorm a bit more! It's a great idea, just needs some tweaking.
1
All of the non-food uses you mention sound excellent to me.
Another possibility could be that it deters phantoms. Trying to dive into them would result in some very soggy and tangled wings. And holy crap, I just want a better defense against these things. They're horrifying.
Rice is also very absorbent when dried. We already have sponges, so that function is out of the question, but there might be some use in the way it expands as it absorbs water. Maybe it could be crafted into a block that, when placed in water, expands into more blocks of varying moisture until they reach a maximum. Not sure what it would be useful for, but it would certainly be interesting.
EDIT: I'm updating the original post as we go.