For my first question, is it cool to give positive trust for transactions that I am not personally involved in?
Yes, in my opinion.
You can give positive feedback to someone without any need for their to be any related trade at all..
Look at
The Pharmacist's positive feedback from DT.. Whole bunch of them with no trades..
I also think that just because you are on DT does not mean that you have to tag every scam you see, or any. Don't feel pressured to leave any feedback that you don't want to..