Glenn Herbert

I would say insulating firebrick for the riser of a core would be fine, getting hot faster than hard firebrick, but concur that hard firebrick is the best material for the firebox.

Look up youtube videos on making a shower floor pan. We had to replace an incompetent professional's 10-year-old floor in a 6' x 8' custom built shower which had rotted down into the floor joists. We rebuilt it with a proper sloped subbase, membrane, and tile bed, laying standard tiles on floor and some walls. It is still working well a dozen years later.

If you need to replace or shore up framing, setting the new top a couple inches below original level will make it easy to match the floor outside the door.

For the radiant floor temperature question, the water in a radiant system is generally around 90 to 110 F. This is easy to achieve with the normal controls for such a system, including a (nonelectric) thermostatic balancing valve that keeps the same temperature in the circulating piping no matter the source temperature.

I built an alternating-tread stair to a small loft 30 years ago, and for several years recently we used the loft as a bedroom. Used regularly, it becomes easy, though you would want a good handrail to hold (we used a hanging knotted rope because the stairwell was so tight). A shed stair with no-slip treads that are deep enough for good footing should work for a shed used mostly by one person.

A snag is a dead tree broken off at some height in a forest, which serves the ecosystem in many ways. Topping a row of living trees would not likely serve similar purposes.

You say "use the remaining trunk as posts for building". Are you talking about using the harvested trunks as part of a building, or building on the trunks that are in the ground? If the trees are currently shading your house, it would seem that building there would have the same issues.

Yes, raising half barrels on a layer of rocks and sealing with cob would work fine. I have not used thin layers of cob on a bench top, so cannot personally advise on the best thickness - many others here can do that.

Cob reinforced with some straw will keep cracks from opening, and a chimney which draws decently will ensure negative pressure inside once the system is warmed up, so any leaks would draw into the bench.

If you make a mass cob bench, you don't need to sift any stones out for the bulk of it, only for the surface layers. In fact, a popular technique is called "cob lasagna": a layer of cob, a layer of rocks as big as will fit comfortably, a layer of cob, repeat until full. Most rocks will be more massive for the volume than clay, which is helpful.

I do think it would be easier for you to make a bell, especially a half-barrel bell, than ducts winding through the mass. It would give much less friction, and thus much less chance of difficult or slow starting, forever after.

Does your barrel have a removable lid? If so, cut out enough of the barrel floor to fit over the riser, seal it to the manifold, and just remove the lid to inspect/clean the riser (in conjunction with the cleanout door at the base of the manifold).

Some people have removed the top of an inverted barrel leaving an inch of horizontal metal, and installed a circle of heavier steel with a gasket fitting into the recess, sometimes just held by gravity.

If you have not built your bench yet, I would advise you to make it a bell (a simple hollow brick box) rather than ducted channels. You will have much less friction, and cleaning will be simpler.

I agree that a simple bell will be considerably easier to build than one with a bench; if you can fit a simple rectangular bell with the right ISA under the stairs, do it. My main thought with the "bench" extension was to get enough ISA under the sloping stairs.

A small chair or two that store under the stairs and can be pulled out when you want to sit close to the fire would work nicely. We have a couple of old wooden little child-size school chairs, very sturdy and perfect for sitting and feeding the fire.

You can try all the tricks you want, but Mother Nature has seen them all and will not be fooled. It is a law of physics that you cannot get more energy/work out of a system than you started with, and you cannot even get all of it back if you do anything with it.