Well done for having it all thought out and described well. This wouldn't be too hard for someone to make, maybe even yourself since it's already based off an existing mob.
For modeling I'd start with the original Wither and add to it using "blockbench" which is a simply free program for making minecraft things.
As for the special abilities, spawning lava would be easy I think. You could have it shoot out a fire charge and tell the game "hey spawn a lava block 10 blocks above where this hits, if it hits an existing lava block." then have it despawn the source block after a second so you get falling lava without ruining the nether with random lava in the air.
For the netherack digging, I imagine it'd work like dragons in Ice and Fire. Blocks have a rating for breaking and just set the wither to break anything around netherack or cobblestone strength.
Finally the magma change can happen once it lowers and you can have it change any block it "walks" on into a magma block!
Love this idea! If it doesn't become a mod of its own it'd be a cool addition to Mutant beasts or mowzies mobs or any boss pack.