All right, I just tried testing out the mount/dismount/rest approach to the problem.
First, I tested it out in the Hinterlands, doing what Natashina did and going around every camp and mounting/dismounting/resting. I noticed something sorta weird - every other camp I visited, when I first whistled for the horse, nothing arrived. If I whistled again, it would show up, but other times, the first whistle would do the trick. Does that happen usually? I've only used mounts one instance before this (ironically, because I didn't want to miss any banter in the game). I had a couple characters make location-specific comments (not banter, though) in this span of about ten minutes, but I reloaded for another session (I checked the thread and noticed that it wasn't necessary to go through all the camps).
This time, I went to the Exalted Plains, an area I had mostly cleared (I'll try a new area like veeia did another time). I mounted, dismounted, rested, and then started criss-crossing the plains for the next hour and twenty minutes. Here's what happened:
1. At four minutes, Dorian and Sera bantered.
2. At twenty minutes, Dorian and Varric bantered.
3. At twenty three, Dorian and Sera bantered again (that was quite hilarious - Dorian asked Sera where she got all her arrows from, as she's got hundreds, and Sera told him from up his ar*e, to which he said it should open a shop if it's so prolific)
4. At twenty eight minutes, Sera and Dorian bantered again (I've never had these two talk so much).
5. And that was it. I was super pleasantly surprised to have four (four!) banters in less than an hour, but the next hour was silent. I tried fast-traveling to another two camps, wherein I repeated the procedure and then walked around again in each instance. Nothing triggered.
I dunno if this helped much. There might be a correlation. I'll try testing more tomorrow. This bug has put my playthrough on halt because I feel like I've missed so much character development, so I'm glad it's an issue being investigated now.