Please note that I am a laymen, not a scientist.
I think you'll find that's true of the majority of us here. If this stuff can be grasped by the likes of me, then there's absolutely no chance you'll have any problems with it.
The chance that it arose in one species after another defies reasonable explaination.
I don't know anything about chance or odds, but I do know that nothing in evolution predicts it happening "in one species after another."
All species with sexual reproduction share a common ancestor. So it only had to happen once - to that ancestor population.
Anyway there's plenty of organisms with intermediate reproductive situations, so I don't see that it takes anything but random mutation and natural selection to produce sexual reproduction. The powerful selective usefulness of sexual recombination has already been demonstrated, so what's missing, exactly? You've got the mechanism, the transitions, and the clear evolutionary benefit.
Can an evolutionary model such as an earthworm or shrimp explain the more complex dna halveing of species requiring male and female?
The "DNA halving" you refer to isn't any more complex in us than in earthworms or shrimp. Earthworms and shrimp are diploid - that is, each of their cells contains two copies of each chromosome - just like we are. Meiosis is normal cell division minus a step. (Sort of.)