GMAT Sentence Correction (SC) | Re: Medicare, the United States government's health insurance

Hi Experts, I have 2 questions:

1) Comma Usage before BUT: Here the usage - Medicare covers the full cost of X, but not of Y. I always see whenever there is COMMA + BUT, then there should be independent clause after BUT. But in this case, COMMA + BUT is used to connect two prepositional phrases. Why is the rule COMMA + FANBOYS should always be followed by IC valid here?

2) ING modifier: I understand in choice B that ING is modifying the previous clause. But the last portion of the previous clause