Nigel you are a very silly person!
Most people ( everyone ?) today - will consider definition 1. to be the current - popular - meaning of conspire.
And, nobody nowadays would use the word conspire except in that way!
And, we are talking now!
It is possible - 80 years ago when the song was written, that the word conspire had a less sinister meaning.
But, I'm interested in how listeners react - or don't react -to that word today.

BTW Pete Seeger is on my side!
A) He changes other peoples songs.
B) He doesn't mind when people change his songs!
So, it's me and Pete against the world!
And, that's fine with me.