Which two grammatical analyses of «فَأَنْتُمَا لَا تَعْلَمَانِ» does al-Nawawī allow?

Narrator Criticism: Teacher Verification, Conflicting Ascriptions and Claims of Hearing

• A categorical declarative negation proving both addressees were ignorant; no interrogation is possible
• Either «lā» is additional (“Do you two know?”), or the interrogative hamzah is omitted (“Do you two not know?”), yielding a confirmatory question
• «Lā» must only be additional; a confirmatory question retaining the negation is impossible
• «Lā» must express prohibition, telling both critics not to investigate the meeting