> Are either of these processes related to the pressure of the mass above? I always thought that was what caused the heat.

No. The mass above plays a role in the sense that it dictates the rates of heat transfer toward the surface from a given depth, but the fact that it's under pressure does not directly generate heat. There are various depth/pressure/temperature related processes that occur, specifically phase transitions at specific depth/pressure/temperature ranges that do change heat content, e.g., the 660 transition is characterized by an endothermic reaction and thus does contribute to the total heat budget.

> 1. Is radioactive decay triggered by something or does it just happen with those particular elements?

Radioactive decay, and its rate, is an intrinsic property of a given isotope. There is abundant literature and details on radioactive decay, but generally speaking, for a given isotope (e.g., ^(238)U, which is one that is relevant for the question), the "rate" of decay is actually a reflection that there is a fixed probability that any given 238 atom will decay, and given an arbitrarily large group of 238 atoms, we can consider this as a rate of decay.

> Is there something besides the insulating qualities of the crust that keep primordial heat from cooling quicker.

Insulating qualities of the entire planet, i.e., rock is not a great conductor, the relative inefficiency of radiation as a heat transfer mechanism (i.e., how heat is ultimately transferred out of the solid Earth), and the radioactive decay all play an important role. For the last bit, it's not just that radioactive decay is adding heat, but also in doing so, it's effectively slowing down the rate of heat loss. Broadly speaking, the rate of heat transfer is related to the gradient in temperature. Thus a reduced gradient (because the interior stays warmer) means that the rate of heat transfer is less. Things get more complicated as we have to think about convective heat transfer in the outer core and mantle, but broadly this point remains true.