I don’t get your reasoning that radioactive decay heat is responsable. OK maybe it’s not the only factor butit has got to be by far the largest contribution to heat geteration. You only have to look at what the natural backround radioactivity all around us is to realise, and a quite back of the envelope calculation with the mass of the earth to realise that you can’t just toss it aside as irrelavent.

neutrinos are defintinely real – the spread in electron energy in beta decay means there must be a third particle shareing the energy – unless you want to throw out conepts like the conservation of momentum.

Most Uranium occurs in the crust as triuranium-octoxide which is pretty stable and has a density of around 8.3grams/cc. I suspect the proportion of Uranium dissolved into molten iron it would not sink to the core as the convective zone stirs it up to much to settle out. The central core of the earth is small compared to the whole volume – most of the heat would be produced in the rest of the mantle.

Fission is hard to acheive naturally – it needs a moderator, a light element such as hydrogen (as in water) to slow down the neutrons, in just the right proportion geometrically to acheive criticality. Uranium metal needs to be very pure with more than around 20% U-235, and no other neutron absorbing impurities to achive any appriciable level of fission on it’s own without a moderator.

The lack of active volcanism on venus now can be put down to the lack of water – plate technonics driven by water present in the crust cause most volcanism on earth. Subducted crust sinks into the mantle and dissolves making it less dense – it then rises and gases dissolved under pressure begin to come out of solution, driving their way through the crust. It is thought now that the major force on plates is being pulled down at the edges by the subduction, rather than being pushed as the mid ocean ridges or friction on the under side from the flow of mantle. The exception are volcanos like hawaii which form under hot convective plumes. It may be that on venus this mechanism is not strong enough now to cause volcanos of this type anymore.

The role of earths magnetic field is one aspect that is still little understood – how does the dynamo maintain itself?