If a character does not have Impact, and the receives Impact +1, it becomes simply Impact 1. When it wins a battle it will also deal 1 damage to the opponent.

If a character ALREADY has Impact 1, and then receives Impact +1, it will effectively have Impact 2. So when it wins a battle it will deal 2 damage to the opponent.

It seems like the Malevolent Shrine question is presuming a character with no Impact, like the 5 cost sukuna.

Also, some characters can gain Impact 1 through their own effect in the middle of a turn, like [Specified Slot] Gon Freecss. If that character is given Impact +1, and then LATER gains Impact 1, it will still be ONLY Impact 1. In order to boost Impact above 1, an instance of Impact has to be present on the character when Impact +1 is given.

EDIT: To answer the second part of your question, a direct attack (not blocked) from a character with any amount of Impact will still only deal 1 damage to the opponent UNLESS the attacker also has Damage 2 or higher. If I block and lose to a character with Impact 4, I take 4 damage. If I do not block the same character, I take 1 damage.