This is a way late response but thought I'd post in case you hadn't found a solution yet (or in case the post helps anyone else). I've spent weeks on researching this topic for my own dilemma, and the best option I found was ferrocement domes. I found my information on ferrocement.com. The shape will provide some tornado resistance, especially if its an irregular dome shape, and it will definitely provide fire resistance. Think giant boulder shape.
I finally found a simple solution to all those gaps in the hardware cloth or chicken wire or metal lath in the ferrocement armature because to hawk and trowel over all those holes with the slurry would be labor intensive. Just spray foam insulation , like an inch thick first, then hawk and trowel on after. The foam hardens to about 53 psi and would keep the slurry from falling through too much. You only need about a 1\2 inch slurry layer which wouldn't be too heavy dried correctly with a 28 dy wet cure to get seriously a 10000 psi strength (see website). It would also give you more labor flexibility with disabilities if you can't get the whole thing done at once. Lastly, Lowe's carries pigments to dye your ferrocement with so you can have a great cob look but ferrocement strength. Ferrocement isn't as green as cob or strawbale, but in tornado alley I think the only way to go.
You should make a little replica first to make sure it works for you. Hope that helps some.
