Fission weapons with arbitrarily small yields are possible, it's just that you have to sacrifice efficiency. Before the CTBT, tests of < 1 ton yield were routinely carried out as part of hydrodynamic bomb testing. The smallest critical mass for plutonium (assuming an advanced weapon design) seems to be about 2-3 kg, giving a minimum yield of 5-6 kt without losing efficiency (and thus provides a minimum cost bound for a fission weapon). The 10 MJ figure I gave was assuming the smallest possible primary, and a staged design. I think at this point we have a very clear idea of what it would take to make pure fusion weapons work (after all, it is in the highest national security interest to do so) and as I said, all the approaches seem infeasible for now and the foreseeable future.

I'm aware of that study. I wouldn't worry about it; the calculations are extremely optimistic and we don't even know if a segregated deuterium layer exists in Jupiter at all.

I think you are missing the point. The point isn't whether you could build a SSB (although, as I said, I doubt it would be of much use in a nuclear ICBM era). The point is whether 'salting' by cobalt would produce a more dangerous device than you could make with just uranium-238. The answer is: No, in fact a cobalt bomb would be safer than a U-238 device.