The first narration is narrated in the book al-Maqtal authored by Al-Turayhi who narrated it from Ubay bin Kaab. The content of the said narration, i.e. that Hussein is a light of guidance to the truth and whoever follows him will be saved from misguidance, is one of those concepts that is found in many narrations about the Ahlulbait, like the tradition of al-Thaqalain and the tradition of al-Safeenah and others. The second tradition is narrated in many Sunni and Shia sources of narrations; Ahmad bin Hanbal mentioned it in his Musnad (vol. 4, p. 174), as well as al-Tirmidhi (vol. 5, p. 324), Mustadrak al-Hakim (vol. 3, p. 177), and many more.
Yes, it is valid without witnesses, as it is not necessary in a marriage contract.
If the two parties agreed on forming a marriage contract between them in such a way that the paid money is considered as dower like the permanent marriage, then the marriage is valid. If she understands that the relationship allows her to have sexual acts in return for a fee, then it is adultery. The same can be said if she understands the relationship as a form of friendship.