Awen:
As far as the actual evolution of sexual reproduction, there are ideas but they are not proven; you can Google as well as I and have already provided links in your post. Here is another:
http://www.youtube.com/watch?v=1w0FiwfyUMM
It looks at the very start of the process (how haploid cells with one copy of each chromosome fuse to become duploid cells with two copies of each chromosome. how this is not irreducably complex as is often claimed, and why this would be advantagous). It is part of a series which means if there are bits you don't get or questions that remain unanswered you can go backwards or forwards in the sequence.
But remember, Google, Wiki and YouTube are not the normal methods of academic learning. You can do scholarly research outside of academic environments but it takes considerabley more time and effort than a bit of surfing.
- Evolution would imply that an organism uses the most direct and simplest means to evolve.
Yes but no. Organisms in a population of organisms that have characteristics that allow them to have a greater number of offspring than is normal for that population of organism will result in those characteristics being more common in sucessive generation of that population. A complex 'accidental' charateristic that allows greater transmission of genes to successive generations will 'win' over a simple pre-existing characteristic. An organism also does not 'know' the difference between complex and non-complex, there is no 'choice', there is just success in passing on genes. Some are better than others.
- Sexual reproduction in humans for example is quite complex so why do we reproduce this way and not assexually as Evolution says the first single cell organisms did?
Try http://www.youtube.com/watch?v=pC8LZlmwCzE&feature=related. Human sexual reproduction is a further elaboration of pre-existing sexual reproductions and as such is slighly more complex than those methods of sexual reprodction that went before it as they were of those that went before them. As the linked video shows, it all started with cells fusing.
Each layer of oomplexity was selected for because it allowed greater success in passing on genes and/or greater variability in the genes passed on so as to allow natural selection more to select from. For example, if an asexual organism can have 1x chances of a new trait, an sexual organism will have 2x chances (this is VERY simplified). So we reproduce sexually as it allowed us to be more adaptable and successful in our environment; some organisms still successfully use asexual reproduction as it wiorks perfectly well for them in their environment.
- Why is the fertilized egg not attacked by the mother's immune system since it constitutes a foreign body (containing male chromosomes) before the placenta is "created".
Two-way process that is not fully understood; progesterone is a immuno-suppresant, the placenta produces another one, and the fetus is active in not being rejected. The placenta is key. But this bio-feedback loop did not happen overnight and each step along the way from cell fusion to eggs to live bearing had advantages.
- How could nature evolve a female member of a species that produces eggs and is internally equipped to nourish a growing embryo, while at the same time evolving a male member that produces motile sperm cells?
Not seperate things. Happened at same time. If a 'male' organism got too out-of-step with 'female' organisms it would not be able to reproduce and vice-versa. Thus over huge amounts of time this arrose in minute steps.
Great questions Awen and some of the answers are not fully understood yet. However, do not fall for the IDist or Creationist dimwittery of assuming just because evolutionists cannot explain everything evolution is fundamentally wrong. We might not be able to fully explain some things but the evidence for evolution, both in fossils and in genes, is incontravertable, and falsifies any literal interpretation of any creation myth.