Is merely wiping away splattered toilet water with water sufficient? And if its location cannot be determined, is wiping still obligatory? Also, is it necessary to change outer clothes if they come into contact with inner clothes that have urine on them, but no trace of it is visible?

The water that splashes back from the toilet onto the person relieving himself is impure water that must be washed off. If it is difficult to determine its exact location, one should wash what is certain or highly probable to be the location of the impurity. If impurity touches something and its location is hidden, one must wash what it touched until its removal is certain. If the area is large, one should exercise diligence and wash what one deems most likely (to be affected). If the area is small, it is obligatory to wash the entire area until its removal is definitively established. If there is certainty of urine drops descending after urinating, it is obligatory to wash the impure area and repeat istinja (cleansing after defecation/urination), and wash or change the part of the garment that was affected by the impurity. As for outer clothing, it is not obligatory to change it unless there is certainty that it has been affected by impurity.