Do skills from Strong Attack Tree have any effect on other skill trees?

Sunder armor is actually applying a debuff on your target when you hit them.

It IS cumulative with fast attacks, you just have to engage with an heavy blow before sticking him to death with your death dance.

Rend is actually an active skill (move), as opposed to a passive bonus, and consequently is not applied during Light Attacks.

Sunder Armor debuffs damage resistance (armor) on enemies you hit with Heavy Attacks.
It does not apply when you attack with Light Attacks, but the debuff remains on the enemy and will affect all damage sources afterwards (including bombs, crossbow, physics, etc).

Sunder Armor: Reduces enemy damage resistance by (5-25)%