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We call this a Lead Lookup Form as opposed to "validation" (since the concept of validation and the onValidate event exist as part of the Forms 2.0 JS API and don't really have anything to do with this).
The answer is actually that neither of these lookup methods can be used from a public-facing form. It's a gaping Denial of Service hole if you should attempt to use any Marketo API for untrusted requests.
What you can build, without the API, is a lookup form based on lead field values or segment membership, since those can be (after a fashion) interrogated by polling for the lead to be associated and loading an LP with representative tokens/content.
Hey Sanford, thank you for the explanation