Could a prospit player be affected by the horrorterrors, or is it only Derse players? I would imagine it would be difficult for a prospit player to be affected, but not impossible; however, we really only have Rose as an example

I think the same. They could likely be corrupted if they were influenced by them- Remember Rose got Grimdark on LOHAC, not Derse. She’d been in contact with them for a while but like… As long as they can reach them through some proxy, anyone is susceptible.