Comma usage correct?

Hello,

I have been reading some journals and I have no idea why they put commas in these sentences. If the commas should be used, please tell me why, e.g., the grammar rule, etc.

1) She also reported a case of lice, for which she had been taking medicine during that time. (Is this b/c 'for' is one of the FANBOYS and 'she had been taking....' is a complete sentence?)

2) The lips were treated to minimize swelling, which may account for the incomplete clearing of sores in this area.

3) This disease may recur, requiring additional surgery, which may no be possible owing to.... (I think I get this--'requiring additional surgery' is a nonessential clause, right?)

4) In the jungle, tissue is selectively damaged, leading to the formation of new tissue. (Why is the 2nd comma there?)

Thank you!

P.S. Can you recommend a clear, thorough book or website that covers grammar usage in general, especially commas? I seem to run across these weird sentences that don't fall under the normal topics covered like: use commas in a series or use a commas before FANBOYS between 2 complete sentences, etc, etc.
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