Glenn Herbert

If you want to do the irregular edged slabs without worrying about the mismatch from routing tongues and grooves, you might try splines. They are essentially a floating tongue that fits into grooves in each board edge. For ease of assembly you would want a tapered profile for the spline, thin at the edges for easy entry and thick in the middle to match the grooves and hold the boards in alignment. Wavy edges could be fastened with shorter sections of spline. I have heard of using a contrasting color spline that would be a decorative touch when the slabs shrink in winter.

For fastening, the advice on screws is the best I have heard; I have plans to use brass square drive screws for a decorative touch that doesn't scream "phillips head"... I would recess the screws slightly to allow for surfacing. The brass would be less damaging to sanders than steel.

A segmental arch like that is a common structural feature in older brick buildings. I am certain it was originally built to support the whole weight of the wall and structure above it; of course, it would be foolish to guarantee that it is still fully capable without examining that whole area of the building.

Is there any settling or other cracking elsewhere in the building? Any later holes cut in the original walls?

Almost 40 years ago I drank instant coffee, and selected Nescafe because it came in a beautiful square glass quart-ish jar with a nice plastic lid (no rusting). I still have and use a half dozen of those for dry storage, and have found that peanut butter jars of a certain size have lids that fit and almost match, as some of the old lids have deteriorated.

I second the recommendation to go with an 8" J-tube system and reduce to your 6" existing chimney. I have a 7 1/2" J-tube RMH  which currently connects to a temporary 6" chimney out the wall and up, only 12' from fuel feed to top of chimney, and it drafts excellently. I believe you will have no problem.

A bell system can be less massive than a ducted mass, but does not need to be. Figure the volume of material in your bell walls, and you can find out how thick they need to be for a desired total mass. You probably don't want thicker than 8" or 9" - it takes heat a long time to get through that much mass. Also, corners will function better if they are quite rounded; the 4" radius corners on my 9" thick bell walls never feel warm to the touch unless it is cold out and I have fired for hours.

If your system is oversized for your house, you will just have to burn less frequently.

Oh, and for the bell, cheap firebrick should work fine. You only need the high grade firebrick for the actual core. You would just want the top of the bell to be at least 6" or so above the riser top... or else you could use a removable steel plate for the top to make it easy to inspect, and also usable for food/water warming or modest cooking.

Building code for masonry heaters calls for a double skin, which is wise especially in a well-sealed house. You would not want any stones (with their mortar joints) to extend all the way from inside to outside face. Two layers of stone with joints staggered, even if they are not independent walls, ought to give enough safety against carbon monoxide leakage, coupled with the fact that a well-drafting chimney will make negative pressure in the bell and any leaks will mostly go into the bell, not out.

Do you already have all of the 6" stovepipe for the mass run on hand? If not (and even if you do), I would advise making a bell instead. For much of the bell you may be able to use local rock mortared together. What is the character of your local stone? Flat, jagged, rounded? Unless it tends to all be smooth and round, you can build a secure masonry box with only mortar to import. Given the relatively constant heating load, a thick walled bell would be appropriate and keep the heat output fairly constant. You would need some firebrick for the top third or so of the bell surrounding the heat riser, as that sees extreme thermal stresses. If you use the traditional barrel around the riser, you can get away with just the core being firebrick or other specialty thermal performance material. What I did (for about 800 sq. ft. in upstate NY) was a brick and cob bell 6' high, with a (not yet built) 6' bench bell extending from the side. The bell walls are about 9" thick, and when heated up, keep warm for up to 24 hours.

For a 6" J-tube, I think you would want a bell with interior surface area (not counting floor) of around 30-40 square feet. A 6" batch box has been calibrated to want 57 square feet ISA, and J-tubes put out heat slower for the same size system so need smaller ISA to leave enough residual heat for chimney draft.
http://batchrocket.eu/en/building#bellsizing
You could make a masonry bell 16" square x 4' high inside, for about 23 sq. ft. ISA, and a bench with interior 2' x 5' x say 6" high above the exhaust opening, another 17 sf ISA, totaling 40 sf. I would want Peter van den Berg to weigh in on this estimate. I think the constant cold climate would aid draft enough to warrant the large end of the range for ISA.

You mention shading from the "June - September afternoon sun"; as that is asymmetric to the solar seasons, I think you would get better results from relying more on trees planted to the west and northwest. They will be in leaf from late May or early June depending on species and climate, until September or October, better matching the shading desires.

In summer, the sun will be so high that it will shine on the north wall in early morning and late afternoon, as well as on the east or west wall. You will have no need to protect from the southwest sun; that will only be prominent in winter when you want the warmth. Porches and overhangs are your friends, when tailored to your specific circumstances. A north-south berm would probably give the best combination of road screening and late afternoon summer sun protection. I would arrange it to not block winter late afternoon sun from hitting your house. There is nothing like your house suddenly getting cold and dark while the hills are still in bright sun.

Soapstone is an ideal material for thermal mass, but is not an ideal insulator. What you want around your riser is an excellent insulator that can stand very high temperatures. The ceramic fiber is that already.

You can perfectly well make an enclosure of brick instead of a steel barrel; the top third or so, at least, should be firebrick so it will last under the heat stress. This can be faced with a solid soapstone layer for additional mass and good radiating properties. You will not get the same instant heating as you get with a bare steel barrel. I would always build in an access panel to be able to inspect and repair the riser if needed, and if well located, this access panel could serve some instant radiator function.

I agree with moving the raised beds off of the concrete. Are there trees close to where you were standing to take the photo? If not, the grassy foreground would probably be a good location for at least some of the beds. I would consider placing some or all of them just at the edge of the concrete at the far side; this would minimize shading from any trees behind you in the foreground. Add some boards or rocks to level the far side of the beds where the ground slopes away, with the near side just touching the concrete so you don't have to worry about weeds there.