The Feldmütze ("Krätzchen") was used in fighting, too. Because of the Pickelhaube doesn't protect against shooting (leather) it was used for protection, when cavalry-attacks were expected.
So in the Danish campaign of 1864 the pickelhaube wasn't used, because no cavalry in sight.
It was telled, that the Pickelhaubes were sent back to the depots.
I think wearing the backpacks in fighting depends a lot of the situation. For a planned attack, the backpacks might be left back at the train, but if attacked on march or on longer march/fighting situations, were the train isn't following, they might have their backpacks on. as well as the mantlet-roll... but this is only speculation by me...I don't know exact facts, but this doesn't really matter these days...;-)