To be fair only Heinrich has thought of a plan and his is kill everybody.
I don't think Annalisa has reason to think that everybody is hostile to her claim. If she does then as soon as the party left to question Anders she would have marshelled all of her guys. Therefore in order to get out of the fortess without a battle [against likely 30 guys] the party needs Annalisa to believe that they are not hostile.
There is also the question of whether or not she is the rightful countess. Julian believes not, Admund also believes not?
? Heinrich wants to kill everybody so I assume he also believes not. Mortus doesn't care, she just wants to avoid a bloodbath. Ulrun?
Also Rufus how big is this keep thing, could Mortus cast fireball inside without hitting members of the party because if [as I believe] she can't her combat ability drops to nearly zero. In which case it would be Julian, heinrich, Admund and Ulrun [with a very small amount of support from Mortus' sleep spell] against 30 guys. That doesn't sound like good odds.
Pleas forgive me if I think avoiding a Total Party Wipeout is worth investing some time in.