All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
As for your first question, you do not have to ask the person whether the saliva of the dog has stained the furniture or not; it is sufficient for you to act according to the principle, which is that that furniture is pure. So acting according to this principle is enough for you, and you do not have to investigate about what you do not know.
As for your second question, you did not clarify the nature of that trespassing to us so that we may clarify to you what you have to do. So please send the question again while clarifying what you want to ask about.
Allah knows best.