It's a shame I never saw any of that in the actual quest...although to be honest, I think I might have missed some content since it was a rather convoluted quest. The only evidence I actually remember seeing on their character was Gaspard's knife being found in the elf servent.
My first reaction was that it was planted, but I don't recall that possibility ever coming up.
I hear ya.
I recently did that quest again, and went out of my way to find everything I possibly could. I found some...interesting evidence.
Gaspard petitioned Celene to work together and not to trust Briala because the benefits of the eluvian network are extensive and he wanted to work with her to secure them, but Briala killed the ambassadors and forged new documents. This is found with one halla statue above the garden before entering Morrigan's workshop.
Gaspard threatened the Council of Heralds lives if they didn't support him when the night is through, and you have to talk to his uncle, and choose the right dialogue option to hear that.
Gaspard gave orders to his men to sneak into the party, just like the Inquisitor was doing, and prepare a bloody coup if things didn't go his way. You find these orders in the trophy room.
Celene knew about the troop movements, having captured and bribed one of his generals and now held him hostage....and naked on her bed, tied to the posts, and was preparing to counter the troop movements and then declare Gaspard a traitor and have him executed. You find out (need four or five halla statues) when you find him in Celene's room and he really wants to get out.
The knife was indeed planted, by Duchess Floiranne.
Celene still has a ring given to her by Briala and has locked it up so she couldn't be blackmailed for still having feelings for an elf despite their differences.
Briala sent an elf servant, possibly to her death, to cover up the fact that she used to be a lover of Celene's.
Gasard hired Ferelden mercenaries to back up his soldiers.
I think it was a fun quest, but you're right, it was convoluted, and getting the full picture requires multiple playthroughs and reading between the lines....extensively because of Orlais's love of double talk in "the game."