Like her father, Elizabeth often jokes. In this case, Mrs Gardiner is asking about whether Bingley really was as much in love with Jane as Elizabeth believes. Elizabeth replies that he was, because he was growing more and more engrossed by Jane. She then jokes about general incivility being the essence of love.

Now, not everyone who falls in love does experience that same kind of engrossment with the loved one as to lead to inattentiveness to other people. But in my experience, it is extremely common and I have often seen that engrossment followed by marriage - and marriages that have so far been successful (I am talking here of course of romantic love in its first flushes, there are many other sorts of love, for example Elizabeth clearly loves Jane dearly but she's never engrossed by her sister like Bingley is by Jane).

I don't see anything in this to make me think that Elizabeth thought the only facet of love, or even the only facet of violent love, was being incivil, she was simply talking about Bingley's particular case and presenting evidence that he was "violently in love" with Jane.

I agree that there's no sign Elizabeth has ever been in love (in the romantic sense) before P&P opens. But one hardly needs to experience something yourself to know about it - the whole point of language is that we can learn from others. And this learning is not limited to merely immediately practical things like "Don't go swimming, there's a crocodile in the river", but also to learning about how others have viewed life, and what the results have been. The view that one must personally experience something to learn about it is extremely limiting and dangerous, for a start I would prefer a doctor who was prepared to learn from others about how to diagnose and treat disease. Moving to affair of the heart, writers have been exploring love for centuries, and exploring marriage and what makes for a happy marriage. For example, Shakespeare's plays and sonnets often deal with love, Johnson wrote on love, Mary Brunton too. And JA herself defended novels in these terms: only some work in which the greatest powers of the mind are displayed, in which the most thorough knowledge of human nature, the happiest delineation of its varieties, the liveliest effusions of wit and humour, are conveyed to the world in the best–chosen language (chpt 5 of _NA_).