The Ghost of the Past is flickering, translucent, and indistinct because Scrooge is trying to forget his past, to make it not exist. The bright halo that surrounds it reminds one that the past can never be completely forgotten.
The harder he tried to snuff out the Ghost's light, the brighter it gets until it's blinding. Scrooge's refusal to accept his past blinds him to the reality of his present life.
The Ghost of Christmas Future being a dark, hooded figure representing the future's uncertainty and his resemblance to the Grim Reaper, which shows the future we must all eventually face: death.
It might seem confusing, that with the present unaltered, Scrooge would have been dead by next Christmas (assuming that the Ghost of Christmas Yet to Come moved only one year forward), but the ending implies that he lived for many more years. However, having a looser, more relaxed lease on life, as reformed Scrooge did, can lead to lower blood-pressure, being less likely to suffer from strokes and heart attacks, and overall improved health. Going further, the cause of his death in the unaltered timeline was not his age as one might have thought. It could have been anything from his high blood-pressure to the stress on his heart, to living in an unheated house and eating nothing but cheap gruel. Marley really did give Scrooge a second chance; by prolonging his life to give him more time to repent.
Alternatively, it's explicitly stated that Scrooge originally died "lying gasping out his last there, alone by himself" on Christmas Eve. His attempts to push everyone away left him completely alone in the end, with no one to take care of him. In the "new" future, the attack that would've killed him probably occurred while he was in the friendly company of Fred (who would probably have invited him for dinner) or Bob, meaning they could fetch him a doctor and/or help nurse him back to health. Scrooge's no-longer-stingy self might simply have been willing to spend the money for the warmth, medicine and care necessary for him to survive an ordinary gastrointestinal or respiratory infection. A man who'd grudged even the cost of enough coal to keep the ink from freezing in his office's inkwells could have easily succumbed to a case of the flu, merely because he wouldn't "waste" a penny on even his own health.
The ghosts (depending on the adaptation) often bear some semblance to either Ebenezer himself, or to the aspect of Scrooge's life they represent: the candle/glowing figure of the past is indistinct yet bright, because he does not wish to remember the past yet cannot forget it; the large and boisterous ghost of the present often either somewhat looks like a "jolly fat" version of Scrooge or otherwise depicts what a man with his life could be doing if he had the Christmas spirit; so why does the ghost of the future not just look "shadowy and indistinct" but rather explicitly like The Grim Reaper? Well, when you're Scrooge's age, especially in that time frame, there's really only one thing you can expect in the future...
Scrooge's laundress and charwoman (cleaning lady) ought to have been familiar to him, at least vaguely. It illustrates the depth of his own former misanthropy and miserliness, that he doesn't immediately recognize people who work for him as soon as they enter old Joe's shop. If he had, he'd have gotten a clue about the identity of the man in the bed much sooner, and the shock of seeing his name on the gravestone would've been greatly lessened.
Scrooge's visitations by all three Christmas ghosts were necessary, their work combining into a multi-step treatment (think IcyHot). The Ghost of Christmas Past helps Scrooge remember how open his heart used to be and the Ghost of Christmas Present shows him things that warm his heart, but it takes the involvement of the Ghost of Christmas Yet to Come to ensure he changes his ways for good.
At the end of the story, Marley is still damned to eternal torment, even after going to bat for Scrooge in the afterlife. Marley specifically states that Scrooge's second chance is "of Marley's procuring." So, if Scrooge had happened to die first, would it have been a Scrooge ghost damned to eternal torment and Marley having Christmas adventures with three spirits after his good damned buddy Scrooge interceded for him? The salvation of a man's soul comes down to a coin flip? Now there's a chilling thought. And that's the point Dickens was trying to make when he told the reader that Marley was dead to begin with, it was too late for him but it's not too late for Scrooge. You can only change yourself when you are alive.
Pointed out elsewhere on this wiki, and pertaining to Scrooge's line how if the poor are going to die, "they had better do it and decrease the surplus population" (which has already been thrown back in his face by the Ghost of Christmas Present). When the Ghost of Christmas Yet to Come shows him the Cratchit family mourning Tiny Tim's death, that line is probably still in his mind. What must he be thinking?
Martha Cratchit is a milliner's apprentice. Millinery is the making of hats, which Dickens and the contemporaries of the time knew to be a very dangerous profession due to the mercury used in the process. The phrase "mad as a hatter" predates the book by decades. Dickens is hinting that, because of the poverty of the Cratchit family and the work they are forced to find, Tiny Tim is not the only Cratchit child due for a terrible fate in the future.