Is it permissible to urinate standing up and then cleanse with two tissues, followed by washing the head of the penis seven times with water, knowing that sitting on the toilet might be difficult due to its uncleanliness?

Urinating while standing is permissible according to Sharia, but it is better to urinate while sitting. The minimum for istinja' (cleaning oneself after defecation or urination) is three wipes, and if that is not sufficient, then the impurity must be removed with water, as wiping does not remove impurity. If you use three sufficient wipes for istijmar (cleaning oneself with stones or similar dry material) and the exiting substance does not exceed its usual place, then there is no problem in wiping with water afterward if the effect of istijmar is pure. However, if it is impure and is excused in its place, then touching it with a hand moistened with water makes its removal with water obligatory, and it is not excused then. The best method for istinja' is to combine istijmar with tissues first to remove the impurity, then use water to remove any remaining traces. If you wish to limit yourself to one of them, water is better. If you limit yourself to tissues, then there must be three wipes along with thorough cleansing.