To address the spray question if you target a model that is engaging you then you will still resolve the attacks on the other models in the spray
For ground pounder, if you are engaged you will not be able to make a ground pounder attack. Currently it is still being discussed on some other points but that is where the discussion currently is
If you hit models with the spray out of your melee but in another melee with other models, do they get the +4 def in melee bonus? Sprays used to just be no melee bonus, but seems to have gotten crazy this edition wording wise.
By default, all models potentially hit by the spray gain the Target In Melee DEF bonus.
Having the Pistol advantage allows the attacker to ignore that bonus for models with which it is in melee:
I cannot immediately think of any special rule or spell effect that allows a spray attack to ignore the Target In Melee DEF bonus for models in melee who are not in melee with the attacker. Other models in melee down the line gain that DEF bonus.
Black penny. Not sure if any models with sprays can be affected by that ability, though.
Thanks!
Yeah, I couldn’t remember a specific example either, and at like 8 AM I was too groggy to feel like checking. 
Firequill has black penny as a field marshal so Deep Borns and Great Old One would get to do this.