Exercise 1: Why the Nonce Check Runs Before Sanitizing Input — Possible Solution
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WHAT THE NONCE CHECK ACTUALLY VERIFIES
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Per Chapter 8, a nonce confirms "a request genuinely originated from
the expected page and user" - it's a check about WHETHER this request
should be trusted and processed at all, not about the specific content
of the data being submitted.

WHY THIS HAS TO HAPPEN FIRST, LOGICALLY
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If the nonce check fails, the request is not legitimate - it may be a
forged, cross-site request that never should have reached this code
handling real form data in the first place. Running sanitize_text_field()
or any other processing on the submitted data BEFORE confirming the
request is legitimate would mean doing real work (and potentially
producing real side effects, like the eventual wp_mail() call) on data
that might not have come from a trustworthy source at all. Checking the
nonce first means an illegitimate request is rejected immediately,
before any of its data is trusted or acted upon in any way.

WHY THIS MATCHES CHAPTER 8'S OWN GUIDANCE
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Per Chapter 8, "a genuinely secure privileged action checks both the
nonce... and current_user_can()... neither one alone is a complete
defense." While this specific form doesn't require a capability check
(since it's meant to be usable by any visitor, including anonymous
ones, given the admin_post_nopriv_contact_form hook), the same
underlying principle applies to the nonce: security checks establishing
whether a request should be trusted at all belong at the very start of
the handler, before any processing of the request's actual content
begins.

WHY SANITIZING FIRST WOULD BE THE WRONG ORDER
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Sanitizing the input first and checking the nonce afterward would mean
the code has already done work on the submitted data (running it
through sanitize_text_field()/sanitize_textarea_field()) even for a
request that turns out to be illegitimate - wasted, and philosophically
backwards, since the entire point of the nonce check is to gate whether
this request should be processed at all before any processing happens.

WHY THIS WORKS AS AN ANSWER
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It explains precisely what the nonce check verifies, why gating
request legitimacy has to logically come before any data processing,
and connects the ordering directly to Chapter 8's own layered-security
principle rather than treating the order as arbitrary.