Monet seemed like an intelligent woman so the capability is there. Before PH we never had a character introduced and killed off in the same arc. In a way, I do like how Oda made a "one arc" character like Monet more significant to the story with this reveal. It also makes DD seem even more heartless because he easily told the big player in his past plan to die for him this past arc. But you're right, I also don't want her to show up and she probably won't. If she somehow survived the heart stab, explosion, and deadly gas on PH, there is no way she can get to DR. She can't fly because she's too injured and she can't stowaway on the SHP ship without 4 haki users noticing. Her reappearance in the story is a stretch, I think we might be worrying over nothing.
The people bowing to Riku could have been doing it out of loyalty and trust but it's hard to have those kind of feelings when they saw him doing the dirty deed with their own eyes. Like Tank who was crying and muttering "What are you doing?..." I doubt he would have bowed to Riku in the previous chapter if he wasn't also controlled that night. The ones still loyal to Riku are probably the ones controlled that night and the people who hate him were probably the bystanders and victims.