I have heard some post i read before that you can use frozen set, crushing blow set, electrocution set as 3 element build but I don’t know if it still relevant toward current version
No. Obsidian can only place Block affix on OH Items.
Not sure. It could depend on the source of the Stun. I would think that if they are resistant to a Stun, they won’t get Stun Locked. Shock Elemental Crit Paralyze is the Stun monsters usually end up resisting, not sure about Stun from other sources, as I haven’t tested it or seen much posted about it on the Forums.
Eternal Items double the values of affixes. it does not double the values of Mythstones or Crystal affix values. you would need to find an Eternal Item with the affix you want, and it will be doubled value. the values are usually a random value higher than the maximum Legend Items value up to the maximum value possible on an Eternal Item.