What are the proofs from the Qur'an and Sunnah regarding the limits of the relationship between a man and a woman engaged to be married, besides the Hadith: "No man is secluded with a woman but the third one is Satan"?

The suitor is a stranger to his fiancée until he marries her. Therefore, everything that is forbidden with a strange woman is forbidden for him, with the exception of looking at her for the purpose of engagement due to necessity. If the necessity is fulfilled, the ruling reverts to the original, which is not seeing her, and he must refrain from looking at her. The fiancée is no different from other strange women except for the promise of marriage. The rulings of khalwa (private seclusion), looking, touching, and speaking alluringly apply to her. Lenience in the relationship between the suitor and his fiancée is a great door to fitna (temptation), for women are the greatest fitna for men.