Glenn Herbert

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.

If using a commercial wood stove damper, there will be no safety issue. A purpose-built damper could give total closure, so the point is worth raising.

I think it depends largely on the character of your climate and soil. Dryer climates and sandier soil reduce the complications and extend the lifespan of earth fast posts. Damp climates and clayey soils are probably the worst case, and softwood posts might rot and fail in a few years. If you can get black locust or Osage orange or another very rot-resistant wood with good installation details, it might last 50 years or more.

A general rule for batch box systems is to keep riser and exhaust sizes the same. A larger exhaust/chimney would probably not hurt, but there is probably no benefit to the larger size.

The door air inlet size is generally a fraction of the system (riser) size. Slots like your drawing shows would allow less airflow than a single equal-sized opening due to edge friction effects. https://Batchrocket.eu includes dimensions and ratios that may help you.

Erika, if you built a batchbox style RMH, you could have a window in the door for fire viewing. Do remember that an RMH, unlike a woodstove, will not be burning all day, but for an hour or two per day in most situations.

If you can get under your house, you can add some block piers in the appropriate location to give extra support. Depending on your climate and how much long-term intense heating you need, you might be able to use a smaller mass that would not severely stress your floor. If it gets warmish in the daytime after cold nights, a mass sufficient to carry overnight and taper off through the day would take care of you.

I must have had strawbales in mind from some other post. Earthbags in your climate may work when well compacted. I am not sure about covering material, especially for edges of steps which would need to be very strong to last.