You know what would actually be really funny?
If Marinette does tell Adrien the truth, and instead of being mad or confused he is actually just SO relieved. Like.
"I knew he was a jerk! Him being a hero didnt seem possible. Him being Monarch? That makes so much more sense!"
And Marinette then asking about the whole thing with Nathalie being his accomplice, he just dismisses it not even mad.
"Yea it makes sense that she was on his side for a bit, she was feeling guilt and wanted to reunite my family, I dont blame her for that. She at least tried."
And Marinette asks if he's really okay with all of this.
"Oh its terrible, but I totally get why you and everyone didnt want this revealed, the chaos it would cause. So, I get why you went this route initially, it must have been hard."
Adrien just being so understanding about it and is mainly just relieved that he knows the truth. He will probably need to process it, but just relieved he isnt going crazy.
Marinette likely just faints from relief