Who the F would send the empress' ex as ambassador?! I know this is a fantasy, still.
Well the manhua really lack context, in the novel it's explain that being an embassador is part of the job of the duke family from the begining.
Remember that time when MC listen to ML talking about his need in women and the fact he was sick of love ?
Well that scene take place while the trash was in war, MC was a proxy and work as an ambassador at that time, this scene was in the empire too.
Plus don't forget that for everyone, it's MC who is clingy. So it's natural they envoy trash to gain her favor, it's not hard she love him for 10 years already from their POV. Well their all wrong but trash is kinda happy to be near her so he stay quiet.