when we sent a typical diplomatic honor guard they responded with an surprise attack, so clearly they don't think much of the idea of a flag of truce. So we send the next diplomat with the assumption that they will kill all present if they think that can get away with it again. They might not like it, but again they opened fire while negating under a flag of truce, no one in the quadrant is going to give an once of space dust what they think.
Once we force them to the table we can ask them why they thought we would need tricks if we were actually hostile.
The alternative would be to send the team in an unarmed runabout. Make it painfully obvious that no threat is present, and bet the lives of the diplomatic team on the honor of someone who just made a fantastic argument that they have none.