Tiaral

Which won't fit through the door. They can be quite large.

Lol. Then we should summon him directly into the large hall, where there's a double or triple doorway. And if the monstrosity turns out to be a cyclops the size of half a room, the bots from the other hall might attack it and flee themselves.

Sharon's body itself must be destroyed.

And he didn't give us anything for that, not even a lousy vial of acid? Is he really a big shot in the spy clan? Then Rails can chop him to pieces, or Goran can stuff a bomb into his body cavity, and it will be like with Mineko. Or whoever casts fire on our side can burn him to ashes. In any case, we'll have to do something like that if the monster turns out not to be a flesh-eater. It could be a unicorn, an elemental, a spirit, or just a humanoid demon.

And I don't think we'll have a chance to discreetly remove the corpse from the palace.

Well, I think we could manage it in the worst case. After all, we definitely have to escape from here, which means we'll take the corpse with us to a place inaccessible to the guards. And then we can split it into pieces and flush it down the toilet in portions. The main thing is that the person is gone and cannot be found, and whether it happened sooner or later doesn't matter. And if we don't manage to escape, then Sharon's body must be left in plain sight.

And there might be a catch in the fact that they have helmets with protection against mental influences.

Very convenient cheat, typical overpowered guard writing.
I didn't pay too much attention to the battle and your plans, so I don't know what's more viable. But I can say one thing - it's not very fun to just put yourself in danger to skip a turn and then be destroyed. Subdue or summon - something can definitely be done. It might not be very effective, I might die in the process, or it might help. If I'm going to die anyway, why pay less?