1. When a model is out in the open, receives a ‘Duck Back’ result, and goes prone, is it considered to be in cover? 2. When a model is out in the open, receives a ‘Duck Back’ result, and goes prone, how are reaction tests handled for that model in the open? Example: The Grath shoots the SA Soldier, the SA Soldier gets a ‘Duck Back’ but has no cover, so the Soldier goes prone. What happens if the Grath shoots again? The Grath and the Soldier can see each other, does the Soldier get a reaction test? 3. What happens if one unit triggers a reaction test for more than one unit? Do all units that can see the unit get a reaction? What determines the order of the reaction?
Ed is swamped at the moment but surely will step in soon to give a "chapter approved", but I think I can help you with that (I already answered you in the FB group).
To be prone is considered as to be in cover when shooting, so a soldier with DB result going prone is considered to be in cover, and if it is shot and the shooter only passes 1d6, the proned soldier will return fire as if he was in cover.
Regarding the last question, just follow the steps from the section "Taking the Reaction Test" and roll for as many groups as there are in sight and reacting. The passing dice of each leader will tell you in which order they are resolved one by one.