1. I don't recommend trying to glue OSB to anything, and I don't recommend gluing a second layer of subflooring to dimension wood subflooring boards. But since that's what you've got I suppose that's what you'll install over.
Your subfloor, as des cribed, doesn't really meet the MIA requirement for double-layer plywood or OSB subflooring, but you can continue with what you've got if you want. You should also evaluate your joist structure to determine whether it meets the L/720 deflection requirement, for which you can use the Deflecto in the dark blue bar above. Pretty unlikely that it does.
You can use the Ditra or a CBU, but I'd favor the Ditra in your case. And that's after you've stiffened up the joist structure. Ditra XL might actually be a better bet.
2. If you use Kerdi System, you don't need a CBU on the walls. If you use a CBU on the walls, you don't need Kerdi. You can use both, but it's not necessary and makes the Kerdi installation more difficult. Up to you.
My opinion; worth price charged.