Yes to the vulnerability and fast healing.
Agreed.
We did a squeezing trait for the bloody bones and something else, but I'm not sure I recall something quite that amorphous. Might be mis-remembering, though.
How about adapting some of the amorphous traits we came up with.
e.g. the Celestial Stag:
Amorphous (Ex): A celestial stag is not subject to critical hits. It cannot be flanked. Because of its jellylike substance, a celestial stag is able to slip through spaces that otherwise could accommodate only a Tiny or smaller creature.
or the Protein Polymorph:
Amorphous (Ex): A protein polymorph in its natural form has immunity to poison, sleep, paralysis, and stunning effects. It is not subject to critical hits and, having no clear front or back, cannot be flanked.
Amorphous (Ex): An infernite's body is essentially living molten lava. They normally assume a humanoid shape but can melt their bodies to flow through or around obstacles before recongealing themselves. An infernite is able to slip through spaces that would normally only accommodate a Diminutive creature at its normal land speed. An infernite has immunity to poison, paralysis, and stunning effects. It is not subject to critical hits.
It might be better to give the the immunities plus an adaptation of the Slime Golem's malleability:
Malleability (Ex): Because of a slime golem's unusual body structure, it can compress its body enough to squeeze through a 1-inch-wide crack. Cracks and similar openings that are 1 inch or more in width do not slow the slime golem at all.
Fluidity (Ex): Because an infernite's body is essentially living molten lava, it can flow through gaps of Diminutive size or more without slowing.
Immunities: An infernite has immunity to poison, paralysis, and stunning effects. It is not subject to critical hits.
I think I like the second option a bit more.