You can just click on tweets and replies to see that, and it'll show up on a timeline if they're in a list. It's not literally invisible to other people.
Right, but nobody would see it if they're not following both users. It's not a matter of literal invisibility, it's that they're presenting it to their audience for a reason.
Quote tweeting happens for the reasons Bigg said. This is part of how interactions on Twitter work. It's not really up for debate?