What should I do about a viscous, transparent fluid that sometimes emerges after urination, and I don't know whether it is madhiy or maniy? And in both cases, is ghusl obligatory upon me or not, bearing in mind that I do not find any trace of it on my underwear? And can madhiy be discharged during sleep?

Using tissues for istijmar after istinjar is not prescribed. What is prescribed is to perform istijmar first, and then use water. If one wishes to suffice with only one of them, it is permissible, provided that purification is achieved and that istijmar involves three wipes. The fluid discharged after urination is most likely al-wadi. There are previous fatwas that clarify the characteristics of al-wadi, al-mani (sperm), and al-madhi (pre-seminal fluid), as well as the rulings related to them, in addition to how to purify al-madhi.