If you enter the Duchal Palace in Chapter 7 and stop the Doppelgangers from killing Liia / Belt, you will get a chance to accuse Sarevok of helping start the iron crisis / bandit raids, and also plotting the deaths of all 4 Grand Dukes. If you are not carrying Sarevok's Diary, however, the Dukes have no reason to believe you, and you immediately die in a plot-forced Flamestrike.
The problem with this is that as far as I can recall, the only in-game reason a player has to visit the Iron Throne in Chapter 7 is if they get instructed to do so by Tamoko. And before she'll tell you that, you have to first agree to promise to do what she wants, before knowing what it is, and then promise to spare Sarevok's life if at all possible. So if we ignore metagame knowledge, the only way to avoid that game-killing Flamestrike is to roleplay a hoity-toity Lawful Good pacifist.
I would like to change this: I feel that the letter from Sarevok to Rashad/Kizska (Duke Eltan's healer), and the letter from Sarevok to Slythe & Kristin, would be more than enough evidence to convince Belt & Liia that you might indeed be telling the truth, and that you don't need to die instantly. So, in order to force Sarevok to show his true colors, you should need:
Sarevok's Diary, OR
The two letters mentioned above, OR
One of the two letters AND two additional letters that directly implicate Sarevok and/or Rieltar as having been very naughty boys.
A fourth option might be possible, involving having a certain number of the plot-related letters collected in Chapters 3-6, but that's a rather lower priority.
Now, granted, a character can always look into the Iron Throne of their own accord, just on the off chance that there would be some evidence against Sarevok there, but since you'd already cleared the entire building in Chapter 5, you don't really have any reason (other than out-of-game knowledge) to think that there's anything new there. I know that, strictly speaking, this isn't UB. But I think it would patch a hole where BioWare kinda dropped the ball, and in any case it's only a minor change that can only ease restrictions a bit without hurting the integrity of the game, and it would make the Duchal Palace encounter a touch more realistic.
Edit: There's no point in "Diary + 1 letter" being an option when the diary itself is enough.