Is it obligatory to perform ghusl for someone who was unaware of its obligation after seminal discharge? And do anterior secretions, other than madhy, invalidate wudu? And do secretions or minor ritual impurity invalidate ghusl?

You must make up the prayers you missed after puberty without the obligatory ghusl. If you do not know their number, then endeavor to make up what absolves your responsibility. Sin is lifted from you due to ignorance, unless you were negligent in learning what is obligatory upon you; in that case, repentance becomes due. Minor impurity, such as madhy and wady, only invalidates wudu and does not necessitate ghusl.