Good post overall...
If your view is that nature always does its best, are you against all medicine and treatment?
After all, nature should be able to treat cancer and fix broken bones since nature is the best at it. Nature gave us type 1 diabetes. We are able to give diabetics a good life by providing them with insulin. Man - 1; nature - 0.
My point is that you have to choose a side. You cannot say nature does its best and we should allow it to take its course, meanwhile we take antibiotics and treat diseases, visit the doctor and undergo surgery. Which is it? I can respect your decision either way, but to me you need to make a choice (no, this is not a false dichotomy.)
What is acceptable to treat by human means and what isn't?
The dichotomy holds true for what you said but these are 2 different things imho.
In vitro is the emulation of a natural process, done because there's no natural way to make procreation happen (b/c infertility).
I back this practice 100% because medicine caters about individual needs, and if an individual can't reproduce the natural way (the best way), the only way that remains is this one. The overall risk is minimal otherwise it wouldn't be allowed.
This is about a single individual or a couple at most, not a statistical population. That's the difference.
By curing people, straightening broken bones, killing cancer you're not trying to emulate anything, because nature does not do that naturally.
As you see, I stated straightening broken bones: medicine doesn't try to link them directly with nuts and bolts because the natural healing is perfectly good, provided that the bones are in the right position so that it doesn't heal sideways.
Still, if someone can't get them reattached, medicine has to find a way to do that.
Statistically, if you remove medicine, natural selection will make individuals better and more adapted. But western ethics are not collectivist, they're individualist. That's why we accept to give up on natural selection of man, so that at the same time each man can live his life at the most, by fixing what nature failed to do in that single individual. This means that there's no natural selection except in extreme cases (disabilities), so as a whole humans will get weaker, see worse (just think people with glasses, in the prehistorical period they'd die because unable to hunt, now they can drown in money and pussy depending on their other qualities) etc.
If I have to pick a side in your post, I'll pick medicine and individualism (which in this topic means having useless kids at greater risk, just to pursue our personal goals/biological needs, even if it means "cheating" natural selection, that's what mine and most people's ethics say, which is opposite to the ones of other guy I quoted in my previous post). I don't think it's really cheating but that other guy considered it this, and indicated this as the cause of the illness, which honestly for me is just tough luck, probabilities are just probabilities, shit happens in pregnancies, it's not a perfect process, no natural process is perfect, even though it was selected through the ages, because the world doesn't work in a deterministic way.
Note: I believe medicine can theoretically do it as well as nature, but in practice this is not gonna happen anytime soon, because it would mean that there's no basic physics research to do anymore, and that we have huge computational power to simulate biological processes, and then the technological capabilities to create the conditions artificially.