Is it obligatory to perform ghusl for someone who remembers a dream and is unsure whether semen was discharged, or predominantly believes it was not semen but something else, or is unsure whether anything was discharged at all? And can semen be discharged as just a single drop?

It is clear that you are afflicted with waswasah (obsessive whispering), so we advise you to ignore it and pay no attention to it, as this is the best cure for it. One who doubts the emission of mani (seminal fluid) is not required to perform ghusl (ritual bath), even if he experiences a wet dream, for the original state is purity. One who doubts whether what exited is mani or madhi (pre-seminal fluid) may choose and assign to it the ruling of one of them, especially if he is afflicted with waswasah. Ghusl is only obligatory when the emission of mani is confirmed and its characteristics are known.