Lightening the mortar will look great and keep those lovely bricks as the feature of the room. Eventually a few other ways to brighten the room could be: lighter coat of paint on the walls & a tall indoor plant or wall art in place of book shelves (if you have another spot for the shelves that is). Gorgeous room - look fwd to seeing the before/after pics when you colour mortar. :)

Lovely fireplace, but back to the two main questions. Will removing the render make a mess? Yes, there is no easy, clean way to remove render, and there is no guarantee that the material underneath the render will be in a condition that you would want to live with, without additional finishing. The fine dust will be the worst as it gets everywhere. Is there sandstone under the render? Based on the age of the house and being of cheaper weatherboard construction I would have thought it unusual to have an expensive, labour intensive sandstone fireplace, unless it was a sandstone veneer on top of structural brickwork, which was commmon in the late 1950's and 1960's. However you did mention that there are sandstone stumps which are pretty rare, as hardwood, brick or concrete stumps were commonly used. Why would anybody craft sandstone stumps? Your close-up photo does look like it may be sandstone, although it could be where the decorative, textured, top render coat has chipped away from the coarser, base render coat. I am curious to know why anybody would render a sandstone fireplace, and the sandstone plinth below the weatherboard? I think that the first thing you need to do is to confirm that the stumps and plinth are actually sandstone, as they would be very unusual for a 1950's weatherboard house. I am not sure how structurally safe sandstone stumps would be unless they were broad and shallow, not thin and tall like most stumps. The curved, upper part of the fireplace is most likely bricks laid as a soldier course, with render over the top, as curved sandstone slab is hard to carve cost-effectively. If the height of this curved part is about 230mm-240mm then it is most likely brickwork. You can also check the recess between the rectangular base and curved top to see if it matches standard brick dimensions. I would be very cautious about removing the current render as I think that it is less likely there is sandstone underneath, and even if there is sandstone it won't be in great condition being a porous stone which has had render over the top. Best of luck, Dr Retro of Dr Retro House Calls

In removing so many internal walls I would suggest you get a professional to check the roof structure to see which walls are load bearing before getting too far down the track. If the walls that you are removing are from floor to ceiling then you should allow for new flooring, and a new ceiling so these elements don't look patched. Apart from these concenrns my only issue is the use of a cavity slider, they are not great in keeping the heat in or out, and can be draghty due to the air gap you need right around the door. I would suggest a pair of hinged, glazed, french doors will do a better job in controlling draughts (and noise), and also allow you to see through to the backyard even when the doors are closed. Make sure that you get your designer/draftsperson to draw all of your furniture into your plans at the correct scale so that you can see that it all fits, and works, and that there is enough circulation around. This is so crucial to get right in smaller homes. Best of luck with your renovation, Dr Retro of Dr Retro House Calls

strip part of the brick and mantle back to orignial if the exposed colour lines up with the room, strip the whole thing. Looks so well crafted not to pay respect to the past