There is that. I suppose you need to have a soul to begin with for the Dementors to injure it.

Other evidence pointing to a conspirator: Milland -- the real one -- swears he has been in captivity for three years. Even if Snape were minimising the amount of time spent in presence of others, and thus minimising his Polyjuice usage, that is still thousands of doses. That he could brew it is no surprise -- although we have not yet found the laboratory we know must exist -- but where did he get the fluxweed?

The only thing I can think of is that a licensed grower has been diverting a steady supply to him. And given how few of those there are, I wonder if that might be a useful avenue for investigation.