So then if I’m understanding correctly it wouldn’t matter if I sent a hammer at a treasury village with my hero on 1% health because the army would still get the bonuses from the weapon and hero attack bonus?
Yes. That's corrcet. The army will get full bonus. But it's not all, specially on treasury village.
In order to steal treasures you need an alive hero.
For example you attack a treasury and you lose only 30% of your army, 70% of your army returns home.
Now if your hero had more than 31% health, he would survive the battle and bring home treasures. and if he had 30% or less he would die in battle and no treasure gets stolen, even tho you won the battle.
Another question, does dismounting and mounting the horse work the attributes the same was when defending as it does attacking?
So for example if I support a village with my hero and then dismount him off his horse does his fighting strength count as infantry defence instead of horse defence?
Nope, No differense here.