What is the interpretation of seeing someone dressed in white before entering the lavatory? Is this related to demonic possession (mass)? And how can one get rid of intrusive thoughts (waswas) especially during prayer?

What you are experiencing is an illusion caused by obsessive whisperings (waswasah), and its cure is to disregard it. You may present the matter to a specialized doctor or a trusted شيخ (religious scholar). It is recommended to supplicate, remember Allah, seek forgiveness, maintain the morning and evening remembrances (adhkar), and associate with righteous people to repel this ailment.