I'm being vague because I'm assuming I'm talking to fellow knowledgeable people about deep implications of 5E mechanics. Aka, I don't feel I should have to prove something that should be plainly obvious to someone well-versed in the topic. I will admit a mistake on the 5%. It's more like 10-15% averaged out across all results. The bonus granted by either drop off sharply outside of the 7-15 result range eventually granting zero at the extremes both high and low.

The game design reason for advantage and disadvantage is to ensure your outcome is representative of the situation and not solely of the dice. It is to ensure a middle result and negate natural 1s for advantage and to ensure a middle result and negate natural 20s for disadvantage. At level 1, an average 10/11 die result should hit goblins and most other CR=level enemies with your proficient weapon. The only particular oddity here is Larian's choice to lower AC but buff HP. That means you'll almost always hit regardless of the situation. The only thing advantage is giving you is a safety net from low rolls. In the game, it also makes things like sneak attack much easier to achieve. Which...

On negating advantage, all you need is a single source of disadvantage. There are tons in the system. Blur is only a level 2 spell, for instance. It's not like you can get 2 sources of advantage, have one source of disadvantage, and somehow still have a source of advantage. It's all negated and is treated like a normal roll without either adv. or disadv. The same is true in reverse.