Person 2「今は……ね。ああ、でも貴方に手を付けちゃうとガーネットに怒られそう。姫様にも色々言われそうだわ。 私よりもよっぽど面倒そうじゃない」
Person 1「本人には自覚は全くないんだがな―― まあ、その様子なら大丈夫だとは思うが、面倒事が起こる前に用事を済ませて帰れよ？」
I was under the impression that 本人 is third person, and if he was referring to ガーネット or 姫様 then he would have used 本人たち instead. In this case then I think it would refer to the person he is talking to, but I don't quite understand. Does anyone care to clarify who he is referring to?
Person 2 jokingly "invited" Person 1 to sleep with them.
1) Why would ガーネット get angry if Person 2 slept with Person 1?
Person 1 is an Incubus who in the past has deceived ガーネット and slept with her, she would naturally get angry if her friend sleeps with the same man she detests.
1) Why would 姫様 get angry if Person 2 slept with Person 1?
Person 1 is also technically her future husband.