I don’t think Medvedev can beat Nadal. Khachanov maybe, but I am not sure if the surface is fast enough for Khachanov. Karen’s form has not been good recently.
Yes, that is the problem. Kachanov has been losing to anyone and his dog... he will be back on track at some point, but maybe not right now. If he shows up with the same for he showed up till now on 2019, there is no reason to believe it would even be competitive. The guy who won Paris masters last year would be a different story, I agree.
Medvedev, on the other hand, is playing serious tennis. And before anyone says his tennis is boring, I can safely say that this is completely and totally true. Yes, it is as boring as fu.. (no, that's not boring), is as boring as he... (not boring again) as boring as watching paint dry. But it is massively effective, as he serves well, actually has a lot of skill and mainly has a very good head, and keeps completely cool in big moments. Historically this is not enough against Nadal, but nowadays a good server who can keep the ball in play may give Nadal trouble. So, between the two,
at this point in time, I guess Medvedev has the best shot.