I was thinking we could use the £350million from the side of that bus.
Unfortunately, that was sent for a re-mark and came back as £18.76.

Back to the subject in question:
I kind of agree with you. I think it's a way of giving the people with ready money a second bite of the cherry. As I said to dd when explaining why I felt like this: If every subject she'd come back 2 marks from the grade up we could have risked wasting all that money on the basis that she'd have gone up in 2-3 subjects.
I know other people for whom risking one paper remarking (at roughly £40) would have meant not eating for them that week. And the fact they'd get the money back is totally irrelevant. They wouldn't have had the money that week to eat.
However: With dd I have over GCSE/A-level put back in two papers, both 2 marks off going up, and both went up. (statistics/further maths)
With the GCSE it was a little bit of a stab in the dark. She'd usually been getting higher on that paper, and so we decided to risk it.
With the further, the teacher had looked at the papers and told her than on one paper she should definitely go up 2 marks (she'd lost 2 marks for accuracy because she'd multiplied out a bracket 2 lines earlier than the mark scheme) and another paper would definitely go up 1 and might go up 2. So we put the first in.
Now to answer your suggestions:
A) 3 marks seems too much to me. That's 6 marks per boundary. There aren't spare examiners to do that really. It would have been, out of dd's 16 GCSEs, I think 6 of them would need to be looked at. (both 3 above and 3 below). And if they were altered, then surely they would need to be looked at again, to see whether the first or the second was more accurate? I see your point, but practically it would cost too much and take too much time.
B) I have 2 thoughts on this. I don't think we knew, but the teachers did. I think less emphasis on boundaries etc did make it more relaxing for me. Otoh dd1 is very hot on such things, I suspect dd2 and ds will pass through the exams without a clue, nor bothering too much.
However by knowing you are up to 3 marks off, then actually it does put the position where you can choose to have it relooked at-so then it does give a second opinion as per suggestion A. Maybe free for people on PP who are up to 2 marks off would be a fair evener?
It's also useful for the teacher to know whether their whole class just missed the boundaries, or whether something's gone wrong and they're way down. Also I can imagine for A-levels you will have times the school says "we don't normally take people on a 6, but as you only missed by 1 mark..."
I think you would also get some people who deserve to go up, especially 5 to 6, who won't want to risk it going down to a 4, even if they expected a 7 or higher.
C) Actually this I would totally disagree with. If they can see the paper they have the opportunity to spot errors like the one above. It also means that someone who does struggle to afford it can look at the paper and say "remark paper 2" rather than putting them all in and hoping (for 2/3 x the price depending on papers) and also be able to see when it's worth it, and when perhaps it's not.
I think being able to see the papers will result in generally fairer marks.
- It means that the examiners have to potentially be able to justify why they gave a lower mark, making them more careful in the first place.
- Because if an error is seen then a school will be far more likely to be prepared to pay for an error they can see, which helps those whose parents are either disinterested or unable to afford it.