Locusts do not grant the AoE experience like heroes do for just being nearby the death of a unit. Instead, locusts gain experience whenever something dies after it has been damaged by the locust within, I believe, the last 5 seconds. Similarly, your mines also grant experience under the same conditions.
Experience isn't multiplied by having more heroes nearby. A minion that is worth 100XP will always give 100XP regardless of how many people are nearby. If two heroes are near that unit when it dies, then I believe both of you get +100XP to your contribution, but your teams experience still only gets the +100.
Abathur, in theory, can cover all 3 lanes for their experience because of his long-range abilities. 2-3 mines can easily blow up minion waves in one lane, a pack of locusts can be in another, and he can be nearby the third lane or even cloned at this time.
