The answers to your questions are in the recent FAQ as I understand:
Q: When a Monster makes a Hurl attack, do you measure the distance hurled from the base of the Monster making the Hurl attack or the target model’s base? (pg 63)
A: Measure from the Hurling Monster’s base.
Q: If a Monster chooses to Hurl, can it Hurl from any point on its base or only where its base touches the target model’s base? (pg 63)
A: Any point on the Monster’s base.
Q: How do you measure which models are knocked Prone by a model that a Monster has Hurled? (pg 63)
A: Any model whose base is touched by a Hurled model’s base as it passed through it knocked Prone and suffers a Strength 3 hit. The only exception are models with Strength 6 or above, who suffer a Strength 3 hit, but are not knocked Prone, as the hurled model stops after making contact with them.
From the third point I guess you can technically knock models in combat prone, assuming the base of the hurled model doesn't touch a friendly model engaged in combat since the FAQ also states that 'Brutal power attacks ignore in the way tests'
In the case that they are both combatants are knocked prone the fight still occurs as confirmed:
Q: If two opposing models in a Fight are both Knocked Prone before the fight begins, do they still Fight in the Fight phase? (pg 40)
A: Yes.
Finally, for the last point prone models remain prone until they move or if they successfully win a fight while prone (instead of making strikes they stand up)
Hope this is useful
Cheers
Not that it is a widely effective maneuver...
