Hook, can you confirm that this is the expected behavior of the March command?
Well, I can tell you what I've seen, and how I understand things to work.
I've seen a lot of regiments using march column. They don't seem to have a problem on encountering enemy units.
When an infantry regiment gets within spotting distance of a threat, which is 500 meters, they will change formation. There is usually time to change formation when one unit sees another. Generally, if they're routing, they're probably already under fire. It *may* be possible that they've been marching for the last game minute within spotting distance but the game engine won't pick it up until the beginning of the next minute, but I don't know the exact processing there.
JMM has said that infantry has time to form square when they spot a cavalry unit 500 meters away charging them so they're never really caught out of formation. This may be different for conscript level troops and fast moving light cavalry, however; I haven't tried to measure it. Cavalry spotting distance is much longer, about 1500 meters, I think, so they can begin an attack before the infantry can see them.
Is all this realistic? Probably not, but it's reasonable behavior within the confines of the game. It just means that infantry will ignore cavalry that's more than 500 meters away, which is probably what you want.
Nafziger in "Imperial Bayonets" says that formation changes are fairly quick, but I'd have to go research it a bit to give you examples. There would definitely be time for infantry to form square, especially since the cavalry aren't in full gallop until they get close.
Hope this helps.
Hook