Is it obligatory to perform ghusl or change clothes if drops of urine splash from the toilet onto the body, given the difficulty in pinpointing where they landed?

Only the area of the body where urine has touched needs to be purified. It is sufficient to wash the part that is most likely to have been contaminated if the exact spot is unknown. As for the garment, it does not need to be washed or changed unless it has been splashed with urine. In that case, the affected area should be washed, or the entire garment if the exact spot of impurity is indistinguishable. There is no need for washing when in doubt about urine having touched, for the default state is purity.