How would mandarin interact with royal seal? It seems both card texts should trigger simultaneously on gaining of the mandarin.
I think this was spelled out in the Seaside rulebook (clarifying Durations), maybe earlier: if two things try to happen in the same time, you get to choose the order. Things trying to happen to multiple players at the same time happen in turn order.
edit: To directly answer your specific case, the Treasures and Mandarin will both end up on top your deck, with the Mandarin either on the very top, or just under the returned Treasures, your choice.
Hmm. If you resolve royal seal first, mandarin goes on top of your deck, then the treasures go on top, because mandarin's card text just says when gained. But if you resolve mandarin first, treasures go on top of the deck and when you go to resolve royal seal's text, it is no longer in play, so would mandarin not go on top then?
If you do get to choose the order of resolution, then I feel the two options would be mandarin goes on top, treasures go on top of that; or treasures go on top, mandarin goes to discard.