Gibb, I'm without suggestions as to the significance of "roll."

To "roll" (to rob a drunk or sleeping person) appears on the West Coast in the 1860s, but it isn't ordinarily used with "down," and I can't see how it would fit into any known text of the song.

"Blow" appears to be the earliest recorded form (1864), but documentation of nineteenth-century chanteying is so poor that the dates mean little.

Like you, however, I'd expect "knock" to have been the original, merely because it's more obviously idiomatic.

Someone wrote in 1934 that "one book, printed in Boston in 1828, reads 'roll the man down.'"

(Right. Show me the book.)