The throat and smoke chamber for any wood burning fireplace, indoors or outdoor, is corbeled masonry with mortar parging, just as the gentleman discusses in the link you posted. Frankly, this is not something you want to try the first time you lay a brick. This is a job for an experienced bricklayer, for structural as well as functional reasons. That being said, there are prefab masonry fireplaces that will give you the shape and size you need to work properly with so you can do the exterior finishing as you choose. I suggest looking into one of those: Isokern FireRock General Shale

“Trendy” is a word that indicates that a style will rise in popularity quickly, over-saturate the market so that people tire of it, and disappear from popularity at a faster pace than a non-trendy item. Whether something is trendy versus classic versus simply part of the fashion cycle has nothing to do with the naturalness of its components. Something such a style of granite can be trendy, meaning that it will burn brightly, but quickly through showrooms, be purchased by many, many people and dominate the fashion so that people get sick of looking at it sooner than other fashion choices from the same era. Trendy items also tend to identify themselves with a smaller number of years. In refrigerators, for instance, harvest gold color was “so early seventies.” Some things start out as trends and become classic. Stainless steel refrigerators are now at that tipping point where they become another classic color, along with white. Almond is an example of a refrigerator color that did not ever trend big. It is often available, though, fifty years after its introduction. It may also be in danger of becoming classic, but it never has reached enough popularity to quite get there. It may yet (or could already have) fizzle out, but would we notice if it did?

Putty, wood filler, caulk, or even lengthy sections of stained string or rope can be used. Not only will these solutions close the holes, but they will also keep drafts from creeping up through the floor. These fillers, which are comprised of putty or paste, can fill in gaps and restore the smoothness of your flooring if applied appropriately.

Bless your heart @M Miller. I did actually add to the discussion. I read what the OP wrote about not wanting to change the wall color, I liked the options Celerie provided and I rolled my eyes at the comment from some one that said unless they changed the color of the walls that room would forever be miserable, like some Grimm Brothers old crow casting a curse on the homeowner. But I’m one of the lucky ones that believes the age of a house has nothing to do with what colors you can or can’t use (unless the OP mentioned this was a period appropriate B&B? An historic house museum?), everyone sees color differently meaning not everyone thinks dark gray is cold or that a cold gray room isn’t desirable, or that a color found in nature like a storm cloud or angry sea is somehow dated or overused or that it limits how you should use it or where or if you’re even allowed to like it.