What, exactly, is Lavos? Is he the core, the bits, the 2nd form, or the shell? I would propose that that is a false dichotomy. Why should we assume that Lavos' shell/2nd form is any less Lavos than the Lavos Core and Bits (I do hope I am not the only one to recognize the really bad double entendre)? They are Lavos and the only thing that ever implies otherwise are a few battles. Given that the Time Devourer resembles the outer shell more than the inner bits, I am inclined to think that any part of Lavos can be a legitimate representation for Lavos as a whole.
Lavos is an alien, after all, should we not expect him (she, they, it, yous, we, whatever) to have an alien biology that is fundamentally unlike anything on Earth? We are familiar with multi-cellular organisms; might Lavos then just be a multi-organism organism, particularly if we take it as the opposite of the Earth itself (as per the Gaia Hypothesis)?
To use a cellular analogy, the shell seems to be the cell wall/membrane, the 2nd form could be seen as the nucleus (or Lysosomes/Peroxisomes), the Lavos Core as its "DNA," and the bits as its RNA. As a whole they form a functioning cell.
Such an analogy seems to be particularly pleasing when one considers the virus-like nature of Lavos. It infects a healthy entity (the earth), feeds off it until that entity dies, altering the host's DNA and its own genetic code, and in the death of both the host organism and original virus it releases new spores to go off infecting more entities.
Just a,
Thought