What is the ruling on secretions that exit after urinating, or performing istinja', or waking up from sleep, which are not accompanied by lust, and do they nullify ablution and invalidate prayer, and is it obligatory to repeat ablution and prayer and wash clothes because of them, and is it permissible to use paper tissues to ensure their absence?

These secretions are impure (najis) and nullify ablution (wudu'). Using tissues for precaution is good. One must wait until their discharge stops, then perform ablution and pray, even if it means missing the congregational prayer. If these secretions exit before or during prayer in the mosque, the prayer becomes invalid, unless you are afflicted with incontinence (sals). If they exit after the prayer, your prayer is valid. If you doubt the time of their exit, it is attributed to the nearest probable time. However, if their discharge is continuous, such that it does not stop for a time sufficient for purification and prayer, then your ruling is that of one afflicted with incontinence.