Abortion is already both legal and morally acceptable in many places, especially in cases the fetus has or is known to develop a debilitating untreatable disease. What if a test were to be developed to detect this condition before the second trimester or whatever the legal boundary is?

I don't think moral realism / moral universalism would claim that whether something is morally ok depends on where (I.e. in what community) it is. Instead, I think "considered morally acceptable" might be an alternate phrasing which doesn't say (much of) anything about meta-ethics.

I understand the concept of a slippery slope, but let's acknowledge the fact that parents routinely make every single medical decision for their babies, because somebody has to, and the babies aren't capable.

In the article, one of the featured patients asked his parents to kill him to end is lifelong torture. You have the privilege of not living with burning skin and relying on hours of medical care every day.

What I'm suggesting is a balance between natural rights and consequentialism. The current situation where we only care about natural rights is also horrific, so there is no ground for a slippery slope argument.

Adults and even children can make choices about their treatment or if they prefer to die (do you think the children in this article should have been given this choice incidentally?) But for babies we should be willing to make that choice for them in extreme circumstances.