So the hiding rules say that if I attack from hidden then I have advantage. I suppose this is more a question of whether Eldritch Blasts multiple attacks are considered simultaneous or whether they are sequential.
If simultaneous it seems I get advantage on each. If sequential then I get advantage only on the first by my reading. What do you all say?
The additional eldritch blast attacks are not said to be simultaneous, say like magic missile, so you'll give away your location after the first one.
Agreed with Plaguescarred. It does not say "Simultaneous" and you are also making multiple attacks (usually) with Eldritch Blast. So only the first one is with advantage. Then the see the other ones comin'!
So no "I cast Minor Illusion and then quicken Eldritch Blast/Scorching Ray!" cheese 
See, I'd say not. Bows are silent, any spell with a verbal component would give away your location before the beans actually went off. The only class that's able to do spell sneak attacks is the Arcane Trickster, and they have a special ability to do that.