Let me say that it's entirely reasonable to read this as disturbing! As anything that plays to non-con kink is going to be. The hint that this particular potion was Dark magic was in there for good reason.

I'll bring up something I've talked about before, which is that in canon "Stephen" routinely does things like lock his interns in cupboards for weeks, shoot people in his audience, and murder hobos then hide the bodies. As a viewer, you get to take that any way from "nobody's actually getting hurt, and the idea made for a good joke" to "wow, this character is a horrible person." Both reactions are legitimate, depending on how you look at it.

For the record, Jon in this fic was very close to stopping of his own volition. He'd already backed off everything Stephen asked him not to do, under the pretense of needing other outlets for his interest. If it got to the point where he had to either force something on Stephen or break character (heh), he would've broken character. It's not that much of a dark!Jon fic (just a surprisingly-good-actor!Jon fic).