@Norbert, post #1
Interlace mode (temporarily or always on) at Amiga power up requires KS ROM patches, there is no way to force interlace without complex hardware modifications. (Intercept and modify custom register accesses)
One possible option is a boot disk that does temporary interlace switch, then resets the system or waits for mouse button.
@peceha, post #5
@Norbert, post #8
@Norbert, post #11
@Norbert, post #6
@Norbert, post #16