You should be fine with that setup. By doing so, your putting the radiators in parallel which halves the flow inside the rads (assuming they are the same), which is actually a good thing for radiators, and reduces their total restriction.
I believe how it is done in the Cross Desk would be two 90° fittings and enough extenders to make them meet.
The size of the tubing doesn't have an effect on performance as long as your above 1/4" ID, though I wouldn't recommend going with 1/2" then changing down to 3/8" for the mobo block. Far as I can tell, the inbuilt barbs cant be replaced.
The card model might have some bridge solution, though really any Crossfire/SLI fitting will do fine.http://www.frozencpu.com/products/14623/ex-tub-1062/Enzotech_SLI_Bridge_Fittings_Set_-_Matte_Black_BSLI-1.html?tl=g30c101s873
The other question 3:
You need a fair bit more radiator space than a single 360mm rad.
I see the name EK and the word Nickel in the same product, you might want to look up on their previous Nickel issues and their current stance on Distilled water.