Something I'd like to check... let's say an attack occurs and attempts to stun the target. The attack includes an interrupt for a second attack. The second attack causes damage but the first doesn't. I assume that the first cannot apply the stun because it didn't cause damage even though the figure took damage between the start and end of the attack? Or does damage from secondary sources count? Similar would be something like relentless. If the target took damage at the start of the attack from relentless but not the attack itself... can a condition be applied?

Only damage from step 7 of the attack counts as damage suffered from the attack.

If the target took damage at the start of the attack from relentless but not the attack itself... can a condition be applied?

No. Only damage from the attack itself counts for applying conditions.