expression
even in the worst case; no matter what happens
Fixed emphatic expression used when asserting that something will not happen or should not be done under any circumstances, often followed by a negative or prohibitive statement.
まかり間違っても、彼に秘密を話してはいけない。
Under no circumstances should you tell him the secret.
まかり間違っても、この書類をなくさないでください。
Whatever happens, please don't lose this document.
万が一 means 'in the unlikely event' and focuses on a remote possibility, while まかり間違っても is stronger and emphasizes that something must not happen even in the worst scenario.
絶対に is a general intensifier for 'absolutely' in both positive and negative contexts, whereas まかり間違っても is a fixed phrase used almost exclusively with negative or prohibitive statements.
Derived from the verb 罷る (まかる, an archaic humble auxiliary meaning 'to do' or 'to be') combined with 間違っても (even if mistaken). The exact historical development is uncertain, but the phrase is now a fixed emphatic expression.