If I can just recap: Digby asked whether there was evidence that Napoleon had given an order to build bridges. No-one seems to have found any evidence but you have been arguing that it would have been impossible to build anything useful in the time. You seem to be envisaging a major road and causeway to match the existing one, which I can well believe to have been impractible but what seems to have been wanted, as expressed by those who were present, was small bridges over the different arms of the rivers.
Digby's book on Leipzig gives an example of what could be done (a civilian writing on the 18th) : 'Prussian engineers arrived and threw bridges over the Parthe. All the trees on my meadow down at the riverbank were felled in order to provide a covering for the wagons that they had pushed into the Parthe. Then they took all the doors from the village to lay over the tree-trunks, even the great door of the manor which took thirty Swedes to lift it. After this was finished the Allied army came up and crossed the river."
Are we to assume that the French army was less ingenious in these matters than the Prussians?