Is the questioner considered pure after performing ghusl for a doubtful state of janabah, then feeling wetness at dawn, and finding yellow discharge in the morning?

There is no ghusl required merely for doubting a wet dream; rather, it is obligatory if you are certain of the emission of semen that necessitates ghusl. This yellowish discharge is not considered menstruation as long as it is outside the time of your regular period, and it only requires istinja' (washing the private parts) and wudu' (ablution).