Glanzmann thrombasthenia: This condition is characterized by normal platelet count but abnormal platelet function, leading to impaired aggregation. The normal laboratory values, including a complete blood count, PT/INR, aPTT, and a ristocetin cofactor activity test, but the need for platelet aggregation studies, point towards a functional platelet disorder like Glanzmann thrombasthenia.

Von Willebrand disease: Although the ristocetin cofactor activity test is normal, which is often used to diagnose Von Willebrand disease, some types of this disease might not be detected by this test alone. Therefore, it remains a consideration, especially given the symptom of frequent nosebleeds.
Other platelet function disorders: These could include disorders like Bernard-Soulier syndrome, although the normal ristocetin cofactor activity test makes this less likely.

Acute leukemia: Although the complete blood count is normal, leukemia can sometimes present with normal or near-normal blood counts, especially in early stages. The symptom of frequent nosebleeds could be indicative of bone marrow infiltration affecting platelet production or function.
Hemophilia B: While the aPTT is normal, which is a common screening test for hemophilia, mild forms of hemophilia B could potentially have a normal aPTT. However, the clinical presentation of hemophilia B typically involves more significant bleeding episodes, such as into joints, rather than just nosebleeds.

Other coagulation factor deficiencies: Deficiencies in factors like factor XIII or afibrinogenemia could present with bleeding symptoms, but these are rare and would typically be associated with more significant bleeding episodes than just nosebleeds.
Platelet storage pool disorders: These are rare conditions affecting the platelets' ability to release their granular contents, which are crucial for platelet plug formation and thus could lead to bleeding symptoms like nosebleeds.