But that's the whole point of having the spokes in tension. It doesn't have to be capable of holding a compression load because as long as the tension in the spoke starts out great enough then the tension will never be reduced enough to cross from zero tension into compression (ie negative tension).

So your line of reasoning gets cut short and since the bottom part of the rim is also getting squashed by the ground, the bottom spokes have to reduce tension before the top spokes increase tension and, thus, the load is primarily "supported" by the bottom spokes (though not by compression, but, rather, reduced tension).

The pretension is like the opposite of a paper weight (which offsets lift forces with extra compression). It just adds a constant to all the forces so that we never get negative tension, but it doesn't change anything else. What's on top has nothing to do with it; The paper is not hanging from the weight.