> You can mitigate tin whiskers with a conformal coat application.

Precisely how effective do you think that'll be when your core packages are fine grid BGA?

> Another possibility is that these are modified CM4 boards where parts were removed and resoldered using lead solder.

This is the only long-term and standard solution I see, including package reballing. However, the fundamental problem with this approach is that the underlying PCB substrate is consumer grade that isn't designed for rework.