Yes. Or at least hint at it, at which point someone will probably volunteer or let slip some information that gives you a rough shape of the causal chain, at which point you know where to dig and pressure further, and eventually convince someone to confess or get a warrant to be sure.

If the prosecuting side has a reason to care that much, it doesn't matter whether it's 10 or 100 people - in fact, if it's 100 people, the original source is in deeper shit because this is now obviously not just personal use, but distribution.

If they are all posting their noise vectors up on xor-music.com, sure. If they have valid reasons for making available a specific 'noise' vector (maybe they can prove it decrypts to something useful), then probably not.

Judges and juries don't need to guilt to be mathematically proved, they just have to be pretty sure.