Installing plywood over 1x6 subfloor in preparation for engineered hardwood
Good. 2-1/2" screws are less likely to strip at the joist connection if that proves to be a problem. You need the screw heads below flush, and sometimes the plywood is hard enough that the screw will strip before it pulls in.
If you're asking for brand names, that's off-topic here. As you're indoors, almost any will do. "Heavy duty" adhesives are solvent-based and bond the best. "Project" adhesives are water-based so they don't dissolve foam, but would still be fine. 1/4" beads on a 8" interval would probably do well. 
This is debatable. I've never concerned myself with decoupling in 30 years of working on homes new and old in Minnesota's wildly varying climate. This site is the first place I've heard of doing so. As long as all layers are stuck down securely, it probably doesn't matter, but I'd be inclined to pinch the original subfloor to the joists with all fasteners to reduce the likelihood of lingering squeaks.
Yes, if by "planks" you mean joists. All layers of subfloor are typically run crosswise with respect to joists. Joints are staggered where appropriate. 
Yes. That's a design decision and doesn't depend on the subflooring direction.