What is the ruling on marriage by Fatiha without witnesses or a guardian, and what should be done after the young man's mother refuses to complete the marriage?

You must repent from having a relationship with a foreign man and cut it off immediately. An agreement to marry without the guardian's permission and witnesses is invalid in Islamic law. The young man's mother accusing you of witchcraft without basis is a sin and an ill suspicion. The mother's objection to her son marrying you should not be given consideration unless there is a valid reason. He must try to convince her. If she does not agree, the general rule is that he should obey her, unless he fears fitna (temptation/strife) for himself, in which case he may marry without her consent and work on pleasing her later. If this marriage is not facilitated, then disregard it, for perhaps Allah will facilitate something better for you.