So right now there is a 2x chance, for 2x events;
Each ancient normally has 0.5% chance to pull a legendary, during a x2 event it is doubled to 1% chance to pull a legendary. Each shard only has 1% chance, so if you pull 100 shards that does NOT mean you have 100% chance to get a legendary, it means you have 100 shards that individually have 1% chance.
As for 10x events, these do not increase your chance to pull legendaries or epics, the chance to pull is the same as normal, 0.5% for legendaries from ancients. What a 10x does is increases your chance to pull a specific epic or legendary IF you pull an epic or legendary with the normal %.
And for the mercy system, Ancient/void mercy system comes into effect after 200 shards. And for sacred it's 12.