The gallon of milk makes it incredibly easy to divide it into quarters
How did you reach the conclusion that liters can't be halved or quartered? You could call 250ml a 'quart' if you wanted to.
Metric butter also has markings on it.
Those medical records should be stored digital anyways, and then it's a matter of a few man hours to tell the computer to convert them.
Each measurement system, US Customary or Metric, has it's own advantages and disadvantages.
Imo metric can do everything US customary can, but not the other way around, because it's missing the simple conversions. But that's it. Everything else is same same but different
Imperial is actually just a couple years older than US customary, it was predated by the "English units".
https://en.wikipedia.org/wiki/English_units