One or two errors:
It was in Burma, a sodden morning of the rains.
Am I right in thinking this is coordination, the comma acting as an 'and'?
No. If the comma could meaningfully be said to 'replace' anything, it would be the ellipted preposition (on).
Was - the main verb- auxiliary
Not auxiliary, but COPULAR . (It would be an auxiliary in e.g. It was raining...)
Burma- noun – adverbial phrase modifier of It-
Not a modifier of any kind, but the object of the implied preposition.
A sodden morning of the rains- coordinated clause- noun phrase- appositive of It (subject of the clause)
Not a clause of any kind, since it lacks a verb! It is a simple noun phrase.
There's no apposition here either. The subject 'it' is expletive, not anticipatory.