Is the wetness found in the underwear upon waking up, which was not inspected beforehand, considered merely a trace of istinja' and not semen?

Your disregard of the wetness and considering it to be from the effect of water used for istinja' (washing after defecation or urination) is correct, because the default assumption is that semen has not been discharged, and doubt carries no weight. Searching and investigating this leads to [excessive] whispering [from Shaytan] (waswasa) and hardship. Some scholars have even considered it among the innovations [in religion]. It is advisable to turn away from waswas and not pay attention to them.