Do clothes become impure and ablution be nullified if a small part of the clothing enters the urinary tract during istinjaa (cleaning oneself after urinating or defecating)?

If part of the clothing enters the urethral opening, it does not become impure unless it is soiled with impure wetness. The default state of things is purity until impurity is proven with certainty. Moreover, this does not invalidate ablution, as the nullifiers of ablution are specific and known. One must be wary of intrusive thoughts (waswas) and not pay attention to them.