Is water that is on the back of the hands considered impure, and does it then render the towel and abaya impure, and then the bag and books, if the moisture in the underwear was madhiy and dried with a yellow color, along with doubt about the source of the water splashed during ghusl?

The whisperings (waswas) have reached a great extent, and the cure is to ignore them and not pay attention to them. Nothing is ruled ritually impure (najis) merely based on doubt. So, the trousers are pure as long as their impurity is doubtful, since their original state is purity. The same applies to everything else mentioned in the question, unless there is absolute certainty about the occurrence of impurity.