I think part of the difficulty is that in my experience and studies, this is exactly how philosophy plays out - by exploring all the corner cases and seeing what the results are in them. If you ignore cases that aren’t just the standard you’ve already lost the argument.
Unfortunately this is often true indeed. I’m not saying to ignore the case of the failure to maintain scenario, but the only thing you can do is to inform them of Church teaching and the call to responsible and selfless parenting decisions. Also, your opponents have no right to back you in a corner of having to pass judgement on someone to “prove” your point.
Conceiving a child who you know will die could be compared to IVF, where several human beings will be created, yet will never be given a chance to live. The Church teaches that there is not an absolute right to biological offspring at any cost, and to selfishly pursue it at any cost is morally wrong.
A couple who is in a situation where the viability and/or well being of their offspring is extremely unlikely, then they have a duty to discern whether or not it is appropriate to continue to try to conceive a child, or if doing so is truly out of a selfless or selfish desire.
Regardless of the discernment – or lack thereof – before hand, once a child has been conceived, parents and medical workers have the duty to care for, nurture, and protect the life of the unborn child in a dignified way with the same dedication that they would for a child that has already been born.
While the pro-choice opponent can argue a lack of prudence on the part of the pro-life parents in their scenario, this does not automatically mean that the parents do not believe that the unborn child is really a person. This is a non-sequitur.
Ultimately, when you are speaking with those who hold a pro-choice position, it can be helpful to remember the words of St. Bernadette: “My job is to inform, not to convince.”
Inform them of Church teaching, and apply it the best you can to their endless “what if” scenarios, but remember you cannot judge the conscience or soul of another.