How the ending of Little Dorrit surprised me

Okay, so Dickens really threw me a head-fake, as I completely guessed wrong about the ending to Little Dorrit; I thought for sure we were heading toward one of those happy but tinged w/ sadness conclusions, with Arthur Clennam dying in debtors' prison and the possibility of a new and more age-appropriate romance between Little (Amy) Dorrit and the man who has for years loved her, John (the prison turnkey). But no - Arthur recovers, LD remains by his side, at last they recognize that they are in love. As expected, Arthur's business partner, Doyce, returns w/ enough profits from his travels (I thought it was to America, but it seems he was traveling in Europe) to get Arthur out of prison and to keep their business in the black - and w/ that knowledge, Arthur and LD head off to the church to get married. Happy ending - though not for poor John - and a bit what's to make of the big, big secret that drove the Italian criminal - Rigaud? - to try to extort money from Arthur's mother? It was so incidental that I thought I missed something; went back and re-read, and even looked up some info about the characters on Wikipedia, and the whole thing is a revelation that Arthur a son from his father's first marriage. I don't see how that is so shameful or astonishing; he never loved the woman who raised him, so what does it matter who was his birth-mother? I still don't get it - but I guess this subplot is what Hitchcock would call the "McGuffin," a plot element w/ no real significance other than to keep the narrative moving. I can see, having finished reading LD, why Bleak House is the more popular of his late, long novels (and Great Expectations more popular than either); LD has some terrific comic pieces and character sketches and a searing takedown of the inefficiencies and cronyism in English government and of the corruption of the financial system - but it's a much more difficult novel to read, with a vast # of characters and some pretty obscure writing at times, and at the heart of it, the title character is so passive, so "good," so willing to sacrifice everything in her life, first for the feckless father and then for what looks to be a hopeless love that we don't really engage with her or care for her - she's too good to be true, or interesting. I also feel it's a shame that, toward the end, CD continued to take shots at the mentally ill Miss Wade and that he has Tattycorum crawling back to the "nice" Meagles family that treated her like dirt. Couldn't one woman at least stand up for herself in a righteous manner?