What's this usage of comma, separating of a list of independent clauses?

Solution 1:

It's not too clear, is it! The original as you suspect contains multiple subject deletions. There's also a missing 'was', and the deletions occurring in multiple predicates gets messy in any case.

Below is the same potted history in note fashion (but consistent grammar):

He spent a disorderly adolescence and youth:

• (1) He never finished high school.
• (2a) Later he landed in some military school –
• (2b) then he turned up in the Caucasus,
• (2c) [where] [he] was promoted,
• (2d) [then] fought a duel and
• (2e) was consequently broken to the ranks. [But]
• (2f) [he was later] promoted again.

He led a wild life, getting through a great deal of money.