Why have scholars not cited a prophetic hadith or a Quranic verse to prove that the wife's sister is not forbidden (haram), despite the existence of hadiths indicating that she is, such as Asma bint Abi Bakr riding behind the Prophet (peace be upon him)?

There is no evidence proving the necessity of physical contact when a woman rides behind a man. What Asma (may Allah be pleased with her) understood from the Prophet's allowing her to ride behind him might have been merely her understanding, not something the Prophet (peace be upon him) intended, as mentioned by Al-Hafiz Ibn Hajar.

Your understanding that the sister of one's wife is a mahram is incorrect; the impermissibility of marrying a woman does not mean she is a mahram.

As for your deduction from the hadith regarding the non-obligation of the niqab, it is incorrect. The incident occurred before the revelation of the hijab, and it is customary for women, both anciently and presently, to cover their faces from non-mahram men.