A $300k fine ($200 per child) isn't sufficient punishment for this. Anti-vaxxers actively maximize the spread of diseases, and to do so in a position of trust like a midwife is intolerable.

Yes, this. She needs to be put away for 10-20 years. Utterly unacceptable.

As mathematically modelled, there are stable strategies that populations of cooperators can use to avoid infiltration by defectors. Are they impractical, or just unapplied?

In my country, midwives don't do vaccines - doctors only I think (maybe for like the yearly flu vaccine pharmacists).

For babies it would be the pediatrician.