Is a small drop of blood that emerges from the anus during Istinja' (washing after defecation) and appears on the garment after prayer forgivable, knowing that my ablution is valid?

As long as the spot of blood comes from the anus, not from the womb, it is not considered menstruation. And cleaning with water (istinja') afterward is correct. The presence of a spot of blood on the garment after prayer does not affect the validity of ablution unless one is certain it came from one of the two private parts, because certainty is not dispelled by doubt. The jurisprudential principle "certainty is not dispelled by doubt" means that whoever is certain of purity and doubts impurity, remains in a state of purity. And a small amount of blood is excused in prayer.