No, even if you do it to bait them into buying T, unless you become Traitor you have no way of knowing if they actually bought it or not. Besides its not really round related info in that way, its just like players arranging to meet at a certain spot in the next round or, similiar stuff. What is important about ghosting Messages is if they affect the current round you are playing, and not your future rounds.
No. Even if you tell them to buy traitor, they may still not do so. They may do it and tell you they will, but you have no way to know if they did. Now if they tell you they bought it for the round you're actively playing through PMs, that would be, as it is going to potentially influence your decisions during that round.