Is speaking with a non-mahram woman without necessity forbidden by scholarly consensus, and what are the sayings of the people of knowledge regarding that if there is no consensus?

Acting upon شرعي (Shar'i) rulings does not depend on consensus (ijma'). If a ruling is deemed more probable to someone, either through a شرعي (Shar'i) proof or by following a trustworthy scholar, it becomes obligatory for them to act upon it. Disagreement is not a justification for abandoning the act. A man's conversation with an unfamiliar young woman (ajnabiyyah) or vice-versa, when not out of necessity, is a gateway to tribulation (fitnah), because the Sharia came to block paths leading to the unlawful and prohibited approaching adultery and its inducements. Some scholars have explicitly stated the prohibition of extending greetings of peace (salam) to an unfamiliar woman (ajnabiyyah) when there is a risk of tribulation. It is permissible to converse with an unfamiliar woman (ajnabiyyah) when there is a need and no risk of tribulation. This includes maintaining ties of kinship with non-mahram relatives through greetings and conversation, to the extent of the need and provided there is no risk of tribulation.