Will a person be held accountable on the Day of Judgment for believing that whoever abandons prayer out of laziness is not a disbeliever, and what is the ruling on him if the opposite becomes clear, and does this belief affect his faith if he is afflicted with scrupulosity (waswas) and uses it to justify doubt in purity and prayer?

The ruling on abandoning prayer and the differing opinions on it are explained in Fatwa no. 104766. There is no harm in following the majority opinion that one who abandons prayer does not disbelieve and does not exit the fold of Islam. The mujtahid is rewarded even if he errs, and the common person imitates one whom he trusts. The cure for obsessive whispers (waswas) is to ignore them and not pay attention to them. Your prayer is valid, and the whispers do not affect it.