I would suggest trying a magic eraser like this: http://www.lakeland.co.uk/24466/3-Magic-Eraser-Blocks

How did it get a gap? When the guy fitted the fireplace did he not use a mortar mix? If so, he could have used the same mix to fill the gaps like you do when you're pointing brickwork. Decorators caulk should be fine, there's nothing in that to bleed in to anything. You will see it though if the gap's quite big. You may need to paint it the same colour as the fireplace. If the guy wasn't wallpapering so soon, i'd advise to get a colour caulk / filler the same as the fireplace, but you don't have time now.

Which do you prefer? The warm creamy hue of the fireplace or the cool hue of the walls? They don't go, but I'm not sure I would want to put paint on the stone. One reason they don't go isn't just the hue, it's the tone. They are both light and this is exaggerating the difference. Is it possible to keep the other walls in the room the pale grey and do just this wall and woodwork a much deeper colour?

I think trying to match the floor is not really a solution. Even if you find the exact boards you will now have a join all the way across in a straight line. The suggestion of black granite or slate is the best one. Are you still planning on having a fireplace in that spot? If not then a piece of furniture is probably the most cost effective solution. I would be tempted to leave the floor until the wall is made good. You also have a skirting gap which is annoying to have to close. I have the same floor as you, I'm too scared to take the kickplate off to check under my island. If I am judging current building standards correctly I'll bet the floor stops short just like yours. It's a big area too.