Is ghusl necessary after seeing a small yellow spot on the sanitary pad that does not resemble the smell of semen, with no recollection of a wet dream? And what should be done regarding persistent doubt about urine leakage, when there is no dampness upon examination?

What you did—performing ablution and prayer—is correct. You were not required to perform ghusl (definition) unless you were certain of the emission of semen, which necessitates ghusl. Do not investigate or search for any discharge. Instead, rely on the default assumption, which is that nothing was discharged. If you doubt the emission of anything, such as urine or the like, do not pay attention to the doubt. Rather, act according to the default assumption that nothing was discharged from you, until you attain certain and definitive conviction to the contrary.