1x Glasscannon & 1x Barbarian do more dmg than 2x Glasscannon (x2.25 vs x2.0).
you can only have 1 kind of any Myth Stone on any Item. so only 1 Endow on your Weapon for +50% WD. from what I remember, it is additive, so is only worth putting on Weapons that do lots of damage or have enough Power Stat to make it worthwhile. depending on the Build, it is kind of a toss up between Endow & +5000 WD. some Builds might have both or neither!
Brutal & Ruptured Mythic are on the same Item (Chest). you can only have 1 Mythic on any one item. so they don’t add or multiply each other.
if you have space for Effective, go for it, but Effective takes a big hit against monsters with Greatly Resist Element for the Element that is Effective against them. this is only a problem against Epic+ monsters with 3+ monster Affixes or maps that give all monsters +1-2 monster Affixes, which increases the chance that they have that Greatly Resist Element Affix. also, it is a +25% to the Element part of the damage, whereas Barbarian or Push the Limit is a +50% to dmg. also, Effective is like Weaken, it only works for the Character it is Crafted on.
here is a great post on how Weaken works with or without Effective, plus how Ignore Resist works.
Bleed Dmg & Lacerate should be multipicative.