To be honest, I love the avenue you are approaching this because its attempting to look at the arguement rationally. While I think the essay is sound in what you wrote, I do however question your basic premise, which is this: The value of life is determined by the quality of life--as seen through the subjective eyes of other people.
You are arguing that "abortion is logical because it does not degrade the quality of life". That arguement has been used in the exact reverse: "killing is logical if is improves the quality of life". This is a dangerous road to go down.
Your central point should instead be: "Is abortion neccessary?"
If it is, you would then have to reconcile with the fact that sometimes killing is neccesary to do greater good, but its still killing. If you are performing an abortion to save a mother, then you have to choose between two evils without a middle ground--in that case abortion is justified as neccesary killing.
Also, is infanticide a form of "abortion"? Because in many third world countries where abortion is unavailable, infanticide is practiced. I'm not judging whether this is good or bad, but I like to bring this up to show the relative hypocrisy some people have with this issue. (i.e. killing is fine when you dont actually see it or have to think or worry about it--which happened in Nazi Germany)