
Tada is a great detective and the manager is a paranoid person. The only thing they have in common is that they both doubt everything around them and they are intelligently suspicious, unlike the detective lady who fell into conclusions far too easily.
From my point of view this is the reason why they are 'immune to Usobuki'
Putting our feelings aside making Yona sit next to him is a great strategy. It boosts the morale of the people and it also shows not to lay a hand on Yona, because the culprit will face the anger of the king.