Long story short - there is work around to synth infiltration attacks that seem particularly glitched in auto failing settlement attacks, even when player is present.

Work around is to target body, console resurrect, and kill again - usually ~10x and quest will update as successful, therefore avoiding the annoying settlement destroyed of most items/crops big.

But turns out can't resurrect ash piles, so it's either gut defenses and go back to machine guns vs using heavy lasers (heavy lasers seem to do more dmg - either because mobs have less energy resistance or does more raw dmg, not sure - and seems to have more range), or live with annoying auto failed synth infiltration attacks again.

All other attack types succeed if you kill every last attacker, but synth infiltration always auto fails unless I use the resurrect workaround - which I can't with ash piles. Annoying.