Just as a mention, in traditional wood burning fireplaces, there is to be a 2" air space between the masonry and any wood or combustibles. Anything you add to this fireplace should be metal, unless there is enough space for a 2X flat, so it will lessen the length of the metal framing. This metal framing should be placed horizontally to frame the front down to within 12" of the top of the firebox. The rest must be masonry, stone or tile. The area around the firebox should be "gauged" so the face of the finished "rough" wall that will receive the tile or stone is flat and in plane with the other surfaces that will receive tile or stone. Running the metal framing horizontal is just a simple way to get a flat wall between the two corners. If you have 1 5/8" of room or more to the blockwork, metal studs will work. If you have places that are only 7/8" behind the face of the wood framing, "hat track" will be the framing to use. If you only have 1/2" in places, RC2 channel will work in those cases.
The rough hearth you have now could be formed to have concrete poured around what you have there, using the original as filler. Make sure you have at least 2" of room from the forms to allow the pea gravel mix concrete to get to all the places it needs to. You could remove all of it and use masonry blocks to build a all new base fore the hearth.
DO NOT use wood framing for the hearth on this type of woodburning fireplace!! Metal fireplaces, yes, there is a metal barrier install in the gap for fire resistance. Over time if a gap forms your type of fireplace, and hot coals get down in the gap, bad things could happen.
In your spaces you have around the masonry mass, if there is anything that needs filling to the masonry a fireproof insulation like rock wool or Roxul(r) will do the trick. The space to the outside you refer to in the write up is not visible to me, but between the insulation, siding and caulk usually seals the outside from the elements. I have gone as far as adding flashing in new construction to isolate the ends of the siding from the stone or brick so the siding does not soak up moisture from that material