Take Level 1 Regeneration leggins and Level 2 Regen chainmail. Does wearing both give any advantage over wearing just the better one?

Unfortunately there's no proper documentation of the game's mechanics. At least I've never found one. So if anybody could enlighten this, would be very helpful.

As equipment doesn't wear off equally, I suspect the game to use hit areas. Hence protection is only provided for a certain hit area (like head, arms, torso etc.). If so, enchantments of protection (e.g. fire) may not stack, but apply only to a certain hit area as well. Still, it's just a guess and I don't know about general enchantments like haste or regeneration.

I too would like to know this.

I managed to accumulate a nearly full set of anti-shock armor, but I haven't noticed much benefit from it, so I suspect that it does only take the highest level into account.

I read speculation though that hits are targeted to specific body parts and maybe protection only applies to hits against that piece of armor. Given that this would make enchanted equipment totally worthless, I am inclined to doubt this.

Resistance being based on the highest level present seems the simplest and most likely execution of armor enchantments and allows an ambitious adventurer to try to get an armor set that includes as many protections as possible.