He has condemned the sin of homosexuality and the sin of murder.

Sin isn't real. He's condemning homosexuals while pretending to care.

The Schachles were certain their prayers to Father McGivney resulted in God curing their son of the condition that almost certainly would have killed him before he was born or shortly after.

Lucky! Not magic.

After assuring the record is in the proper canonical form, it is reviewed by a panel of medical experts, practicing physicians and medical professors in Rome, who aren’t necessarily religious, Caulfield said. With their review complete, “one day in Rome, they get together and cast their votes. Is this a miracle or not, can we explain this or not,” Caulfield said.

Truth is not decided by vote.

Do we have any insight as to how they determined, medically, that a miracle had taken place? Or is it another black box that the church suspiciously does not want to share the contents of?

It's a complicated condition: https://biologyinsights.com/what-affects-the-hydrops-fetalis-survival-rate/

The survival rate for hydrops fetalis is highly variable. Overall, the prognosis can be poor, with historical survival rates often cited as low as 10-20%. However, advancements in diagnosis and treatment have led to improvements in outcomes. Despite progress, the condition still carries significant risks of stillbirth or death shortly after birth.

I would be curious to know more about his particular case.

As for the scales:

the Vatican's Consulta Medica confirmed the cure was instantaneous, complete, lasting, and scientifically inexplicable.

Where is this evidence?

the biological impossibility of genetic reversal is the forensic core.

I would love to see evidence that this is what happened.

It's also a known phenomenon: https://scienceinsights.org/do-genes-change-over-time-what-actually-happens/

Because epigenetic marks are added and removed by enzymes, lifestyle changes can push them in the other direction. In a randomized controlled trial, an 8-week program combining dietary changes, 30 minutes of daily exercise, sleep optimization, and twice-daily relaxation breathing was associated with a 3.23-year decrease in epigenetic age compared to the control group. That’s a measurable reversal of biological aging in just two months.

As for the heat she felt, that is a common symptom of her condition, the thick skin is less efficient at releasing sweat. I find it an interesting case, but I am still not convinced a miracle took place.