In brief, should I insist on marrying a girl whose mother is trying to sabotage the engagement, knowing that her guardian approves? And should I waive some of my rights if the girl's maternal uncle intervenes after reconciliation between the mother and her daughter?

If the girl is pleasing in her religiosity and character, then be keen to marry her, according to the Prophet's (PBUH) saying: "So win the one who is religious, and you will prosper." Religiosity is more worthy than wealth, beauty, and lineage. If the disagreement is due to her mother, try to appease her by compromising on matters that do not affect your religion or honor. And know that the guardian of the girl is her father, then his father, then his sons and brothers, and the maternal uncle has no guardianship over her except by proxy.