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.
To say nothing of the Ghost of Christmas Future being a dark, hooded figure representing the future's uncertainty and his resemblence 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, 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 course 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. 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.
There is a layer of subtle terror in Zemeckis' depiction of the Ghost of Christmas Future as an incorporeal shadow. With this spirit Scrooge has no companion to serve as a guide or confidant to share his thoughts. At the beginning when Scrooge talked to the charity men, he said he wished to be left alone, and now he truly is alone.
In the 2009 Disney adaptation Scrooge takes the pennies off Jacob Marley's eyes. The pennies used as fare to get to the other side to face judgement.
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...
In the 2009 version, we have two kids named Ignorance and Want. Ignorance seems to spend his life in and out of jail, while Want, a girl, possibly goes down another path of ill-repute. The Brilliance comes when Scrooge is dealing with the Ghost of Christmas Yet To Come, and sees the two people picking through his belongings once he's dead. They look a lot like Ignorance and Want.
Want also gets put in a straitjacket, suggesting a mental asylum. Some venereal diseases—syphilis, particularly—have that effect on the brain and, by extension, the mind.
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.
Why was Scrooge surprised to find out that he would die alone and hated? He seemed aware of how poorly people thought of him.
Why was Scrooge, of all people, given this special chance for redemption? Especially if, as Marley claimed, Scrooge's chains (sins) were heavier than his.
Scrooge was given a special chance because he possessed the potential to do great good. It was as much for the sake of everyone who was subsequently uplifted by his redemption as much as it was for Scrooge himself.
That's true although Marley specifically states that Scrooge's second chance is "of [Marley's] procuring." So you only get ghosts to visit you if you have someone on the other side willing to approach the powers that be and plead on your behalf. So Scrooge was saved by The Power of Friendship as much as anything else.
As for why he seemed surprised, it's one thing to think to yourself, "nobody likes me and I don't care", and actually seeing just how little your death affects anything, and how, if anything, his death more or less makes the world BETTER, since he's not around to ruin people's good cheer anymore.
Of course, knowing something and seeing it for yourself are two vastly different things. Things we know have relatively mild impact on us, but things we witness have much greater impact. It's pretty obvious that Scrooge, much like most of us today, never thought that he would see his own dead body buried in his own grave, and probably without so much as a coffin to boot. It was being forced to confront the full reality of the situation that shook him up so bad.
I confess I've never read the original story myself, so it is possible I could be thinking of something else, but isn't there a line in the beginning explicitly stating that Scrooge considered himself a pillar of the community? Bit of a kick in the teeth to have one's self image shattered like that, really.
No, there's no line like that, Scrooge has no illusions about what people think of him, he just didn't care in life, but that doesn't mean he expected people to joke and laugh about his death and looting his corpse. He might have been less shocked if there was simply grim indifference to his passing. Also, keep in mind that the story's point is that Scrooge wasn't evil or uncaring by nature, he was originally a good man who had been worn down and twisted by a hard life and his own greed. In some dark corner of his soul, Scrooge didn't really want to be remembered like that.
What was Tiny Tim going to die of? While his family was poor, they didn't seem to be suffering of malnutrition, and there's no mention of Scrooge paying for any life-saving medical treatment. So, his family having more money saved him? How?
Back in the second half of the 19th century because there weren't any health insurances available in England, doctors were very expensive, too expensive for a family who had to survive on Bob's slim salary and possibly of what his wife earned as well. With Scrooge's support on the other hand, they were able to afford proper doctors and medicine to help Tiny Tim.
Scrooge resents Bob's request for a day off on Christmas, and it's treated as the act of a cold-hearted jerk. Yet both his journey with the Ghost of Christmas Present and his actions immediately after waking up on Christmas morning show that London's markets are all open for business. So why is it cruel to ask an accountant to work on the holiday, but perfectly all right to expect the city's grocers and butchers to be on the job rather than home celebrating?
There's a difference between a job that involves money-lending and a job that involves providing food. Also Scrooge would most likely hold long hours where as the grocers and butchers and most of England would be celebrating at night, the evening with parties and a christmas dinner.