What is the ruling on contaminated water droplets (from urine and feces) splashing from the toilet onto the body, and is it obligatory to wash the part of the body suspected of being touched by these droplets, especially given the difficulty and the presence of waswasa (obsessive whispering) concerning this matter?

The spray that splatters from the water used for istinja' (washing after defecation or urination) is not considered impure unless it has changed due to impurity. If you doubt its impurity or its reaching the body, then discard the doubt and base your action on the certainty of purity. If you are certain that impurity has reached a specific part of your body, then pour water over that spot only, and that is sufficient for purification. Some scholars are lenient regarding a small amount of impurity. You must cast away obsessive thoughts and not pay attention to them.