I think you -could- generate a network of cells where cells are independent and the communicator doesn't know the cells - broadcast your radicalisation on radio

I think you -could- generate a network of cells where cells are independent and the communicator doesn't know the cells - broadcast your radicalisation on radio and let the cells organise themselves. At least one cell member knows about the communication method but since it's broadcast, I think that's safe (in this context.) But practically there's a whole bunch of problems as you say - you've no idea how many - if any - cells exist, they can't coordinate on which one picks up a particular target, the communicator is obviously at risk from being tracked down, how do you tell people where the radio broadcast is, authorities can obviously listen in once they've found the broadcast, etc.