They are similar but no the same; downarching would be simply arching your arch at the wrist in the downswing with no big chance in the left arm wedge. Turning the sweet spot off the plane is related more towards teh whole left arm wedge turning off the plane.
Is it possible to have ideal impact conditions by rotating the l-a-wedge only? Or is downarching necessary to create ideal impact (tour average) conditions?
Would it be necessary to downarch more with shorter clubs than with longer?
I would believe it's more of the opposite, you need to turn the sweetspot and the club off the plane more so with a lower lofted club and thus is also the reason why you need to "hold the twist" more (NSA reference) with them. Downarching is really similar to the twistaway.