True. We aren't sure of that. But it has been made clear that Dragons cannot be converted. If that was even a concern, it suggests to me that it isn't a mechanic specific to Gwen's character. There must be some wider risk of conversion if it needed to be said that Dragons were immune. Maybe it doesn't mean that all Riders are susceptible, but that is what I believe.
We know that if a Rider is killed, we lose the Dragon, too, to suicide and I believe that if a Dragon is killed, we keep the Rider, but they convert to evil.
I think that there were 3 bad guys to start, two of whom were wolves, one who was something else. That something else could read as village to a Seer.
A rider character whose Dragon was already dead at the start could be that something else. That is what I'm saying.