These descriptions are pretty straight forward.
You heal 20% damage over 10 seconds. It does not say after, nor before. It says over which here means encompassing the entire period of 10 seconds. You heal a total of 20% of the damage you have taken over 10 seconds. That part is completely straightforward and proper english.
The perk that gives 5% attack for every enemy within 8 meters. It does not give additional perimeters, and likely doesn't need them. You seem to be complicating a really straightforward skill. Does it have a cap? Maybe, I doubt anyone would know.
You can not stack identical masterwork items.
For exact mechanics, this game is pretty new so feel free to test some things if your especially curious. Some of the finer details on some skills are still up for debate, but its not hard to join in this by observing how these abilities effect your character and letting others know. Like, recently a player discovered that its not exclusively health damage which is healed by this skill. If your barrier is hit it will heal 20% of the damage as health. I think it more strange if it doesn't heal through guard since the skill doesn't exclicitively say it needs to be health damage, I think. There's also the thing where identical items don't stack, but slightly different items will. Its very likely a 5% cooldown and a 7.5% cooldown will stack for example.