Zarya's damage scales with Energy with a 1%:1 ratio. Once at 100 Energy, she deals double damage with her Particle Cannon. Her primary fire goes from 95 DPS to 190 and her alternate will deal anywhere from 45 to 90 damage.
She won't heal teammates for more but will damage for more. If an allied Ana uses her grenade on a teammate and you shoot an ally while they're under the affects of the buff, it will do the same amount of healing as it would if you were the one who threw the grenade. The buff doesn't stack.
Best Answer
Ana's damage-over-time ticks are really fast, faster than her rate of fire.
Unless she's targeted by her own ult, her over time damage will have resolved before she could fire another round.