Is it Islamically permissible to proceed with the case and claim the remainder of the mother's share in the inheritance, or is this considered a breach of the previous agreement with the maternal uncle, given that a signed acknowledgment of receipt for the agreed-upon amount exists?

Since you reconciled for the mother's share in exchange for the mentioned amount, this reconciliation is valid and enforceable, and you have no right to revoke it, because reconciliation is permissible except for a reconciliation that makes lawful what is unlawful or makes unlawful what is lawful, and Muslims are bound by their conditions.

The people of knowledge have stipulated that if reconciliation is concluded according to its conditions, everyone must abide by it, and it is not permissible to invalidate it. Therefore, it is not permissible for you to continue the case and demand the completion of your mother's share, as long as no one coerced you into the reconciliation, but rather you hastened to it of your own free will.