Reply to "Comma usage correct?"

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?)
No. This comma is to separate a non-essential clause modifying 'a case of lice.' The information in this clause is extra; it is not necessary to identify the case of lice.

'For which she had been taking medicine...' can also be expressed as:

...which she had been taking medicine for...
...that she had been taking medicine for...
...she had been taking medicine for...

2) The lips were treated to minimize swelling, which may account for the incomplete clearing of sores in this area.
This comma, too, comes before a non-essential clause. This clause, however, modifies the entire clause that comes before it, not just one noun.

In other words, this extra information -- 'which may account...'-- refers to the fact that the lips were treated to minimize swelling.

3) This disease may recur, requiring additional surgery, which may not 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?)[
Yes, this too is a non-essential reduced adjective clause, or, adjective phrase.

The complete clause would be, '...which leads to...'

The links Jerry gave are good ones for the most part.

I do, however, disagree with this rule on the OWL site:

13. Don't put a comma between the two verbs or verb phrases in a compound predicate.

Incorrect:We laid out our music and snacks, and began to study.

Incorrect:I turned the corner, and ran smack into a patrol car.

Commas are used to indicate pauses, and in any case, to facilitate understanding.

Commas in the sentences above would not be incorrect.

And about Darling's site, here's the link that goes directly to commas: