Glenn Herbert

Brick has enough texture that even clay-sand mortar should seal it up in bell walls. Stone that has textured surfaces and shapes that would allow stable dry-stacking can also be reliably clay mortared. Smooth round stones would depend entirely on the clay between them remaining sound and not cracking, and I would never make a single-skin enclosure with them. I would feel good about making an inner skin of a couple inches of cob, being sure to seal up any cracks that develop in drying.

If you are in a mild climate, you really don't want a big high-mass heater. A small heater with minimal mass around it would be optimal. If you already have a modern relatively efficient wood stove, it probably doesn't make sense to scrap that and build a new RMH. I would do as you suggested and add a bunch of masonry around but not touching the existing stove.

For mortar, building code in the US requires refractory cement (around $50+ for a 50 lb bag of powder) for all heat-exposed and combustion-product-exposed areas. This would not include exterior bell facing stone or brick. Many builders recommend a fine clay-sand mortar (explicitly prohibited by the code) as the best material.

Clay is easy to dismantle and reuse if ever desired and easily cleans off of bricks. It does not cement the masonry together, just beds units firmly.

Refractory cement is strong and permanent, but may destroy the bricks if you ever try to take it apart. You decide which suits your situation better.

The choice between brick or stone or other exterior material is strictly esthetic and practical, though denser materials hold more heat for a volume. The idea of buying stones boggles my mind... of course I do have a large steep creek with unlimited stone bedding, and any digging job turns up a good number of stones.

If a cabin with a RMH burned down, somebody did something very wrong, which can happen with any heat source. A properly built RMH poses much less fire danger than any other kind of wood burner.

If you are using firebrick already, it only makes sense to use that as the first layer in the ceiling above the riser. Firebrick will be the most durable material you can get. Cob may be durable depending on the specific character of the clay.

A layer of wood ash might work to separate the cob from the inner layer of bricks if you could keep the ash against the brick, but that is not a practical thing to do as it would just fall down in a heap, not stay in a vertical layer.

For surface bonded walls to be at all structural, they need to be coated on both sides. I have not depended on surface bonding for long-term use, just to stabilize walls before filling cores.

If filling cores, I would also bond first so that there are no concrete drips making the surface uneven.

Not generally. The firebricks or inner layer of masonry of whatever kind is usually mortared, either with refractory cement as code dictates or with clay-sand mortar as many builders recommend.

Another possibility for the combustion core is a steel frame that compresses the bricks together,often with a thin layer of ceramic fiber in joints to seal and absorb irregularities. This is sometimes called a tension frame (I guess because the steel is in tension), but the important part is the compression of the bricks.

I built my house foundation in upstate New York nearly forty years ago with dry-stacked 8" block, cores filled with concrete and vertical rebar about 2' on center, bond beam at top, backfilled with bank run gravel and clayey soil. It is on a hillside, so two sides are full height block and two sides stick framed.
During backfilling after construction and surface bonding for water resistance, a pickup-sized dump truck full of gravel slid about 6' down the slope and hit the free end of a 25' section of wall (with a couple of one-block buttresses). I saw the wall displace 3 or 4" at the top, and when we pulled the truck off the wall after shoveling it all out, the wall sprang back to plumb with no visible damage. It has been sound ever since. The recent addition foundation I built has #3 rebar 2' o.c. each way and is nice and sturdy, backfilled before floor framing was built on it.

Keeping the courses level is important; in a couple of places I let a hump build up for a couple of courses, and it spread and magnified itself with every course thereafter until I shaved some blocks to bring the coursing back to level.

If you don't have bond beam blocks on hand, it is not hard to knock out enough of the top webs of a course to set rebar for bonding. It does not need a lot of concrete placed continuously around the rebar; the broken webs make good rebar chairs. A circular saw with a masonry blade can cut an inch or so deep to make it even easier to knock out rebar space.

I fabricated a 4" stainless steel guillotine damper for the bypass in the RMH I am building for my best friend, because I want no flow from the top of the bell unless bypassing. There are few other situations where a total shutoff damper would be needed.