Is a slight trace that remains after cleaning with water (istijmar and istinja') and what follows it considered Islamically excused? And is using tissues to re-verify that no trace remains, even if a slight trace appears on them, considered an act of extremism (tanattu')?

The best method for Istinja' (cleaning oneself after relieving nature) is to first remove what is on the exit point with tissue or the like, then wash it with water until the area becomes rough again as it was before the impurity. In Istinja', strong presumption is sufficient. Further inspection and searching after that is an extremism in religion and a cause for whispers (Waswas). If traces of impurity appear on the tissues used for drying, then the area must be washed until that trace is removed, even if it is little, as a small amount of urine or faecal impurity is not excused according to most scholars.