Updated.
For the magma missiles, the original damage was 2-8. A magma hurler's magma rocks deal 3d10+8 plus 1d6 fire. Both are 4 HD and Medium. So perhaps set the infernite's to 2d4 plus 2d6 fire (the same fire damage as its heat)? Although this is significantly less than the magma hurler, the infernite has other benefits (such as heat), while the magma hurler is a one-trick pony.
Water causes 1d10 points of damage per gallon poured onto an infernite.
Infernites are able to manipulate their fluid bodies to flow through cracks and around obstacles. They can pass through any cracks or holes that are at least 6” across at their normal movement rate.
Also, when body material such as hot coals or lava presents itself, infernites have the ability to bond with it and regenerate 1 hit point every other round given no interruptions (see Ecology).
Yes to the vulnerability and fast healing.
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.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.