This is just an off-the-cuff thought, but on the issue of Suggestion: What happens when the Suggestion conflicts with other orders? Not past orders (like the guard at the gate), but, in the case of the OP's example, what if the dragon just says, "Where do you think you're going? Get over here and fight!"
Getting someone to leave the library when there's no one to contradict the Suggestion is easy, but getting someone to leave a fight when a superior is right there giving them orders? That seems a lot iffy-er. This isn't a Dominate spell.
And then there's this part of the spell description: "The suggestion must sound achievable and not involve anything that would obviously deal damage to the target or it allies." Leaving the dragon sounds like it would "obviously" lead to damage to the abomination's ally, the dragon, since a bunch of adventurers just walked in loaded for bear. (If it happened at a dinner party, on the other hand, there would be no obvious expectation of violence.)
Even the "achievable" part of that phrase can have an impact. If your boss will brutally beat you if you disobey orders, and is right there watching you, leaving the room is not really an "achievable" goal.
My feel for the spell is that it makes something you suggested seem like a reasonable thing to do (even without that caveat in the spell description), and, absent other factors, causes the target to do that reasonable thing. However direct orders almost always override "doing a reasonable thing", because orders by their very nature demand ignoring the reasonable in favor of the unreasonable.
Now, if the Suggestion was, "Ignore the dragon; he's not the boss of you.", then the outcome is more likely in the players' favor. At least until the abomination decides he just likes a good fight, and doesn't have to be held back by the dragon's whimsy.