Does the discharge of secretions, of which one is unsure whether they are semen or not, after waking up from sleep accompanied by pleasure, invalidate a compensatory fast, and does the discharge of pre-seminal fluid invalidate the fast?

If someone doubts whether what exited them is madhy (pre-seminal fluid) or maniy (seminal fluid), then according to the Malikis, it is considered maniy. The Hanbalis have a detailed explanation. The relied-upon opinion among the Shafi'is is that the person has a choice: either consider it maniy and perform ghusl (ritual bath), or consider it madhy and ghusl is not obligatory. This is the stronger opinion. What you did by performing ghusl and considering the discharge as maniy is a correct action in accordance with what is right. As for fasting, it is valid in any case, whether the discharge is considered maniy or madhy.