Exercise 1: Narrator at the Sign-In Screen and Chapter 4's Secure Desktop — Possible Solution
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WHAT MAKES NARRATOR-AT-SIGN-IN POSSIBLE
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Per this chapter, "Narrator can be invoked directly from the sign-in
screen itself, before any user has authenticated at all - this works
because the sign-in screen runs in its own isolated system session."
Nothing about a specific user account's own settings governs whether
Narrator is available there, since no account has even been selected
or authenticated yet at that point.

WHAT CHAPTER 4's SECURE DESKTOP ESTABLISHED
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Per Chapter 4, UAC's consent prompt runs on "a separate, isolated
desktop session that ordinary applications... cannot programmatically
interact with or dismiss" - a session that exists independently of
whatever is running on the regular desktop, specifically so it can't be
tampered with by anything running there.

WHY THESE ARE THE SAME UNDERLYING PATTERN
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Both the sign-in screen and the UAC secure desktop are genuinely
separate system-level sessions, existing independently of any specific
user's own logged-in desktop and its running applications. Just as the
UAC secure desktop can't be reached or manipulated by programs running
on the ordinary desktop, the sign-in screen's own session - including
whatever accessibility tools it offers - doesn't depend on, or get
affected by, any particular user account's settings, because no user
account is active in that session at all.

WHY THIS DESIGN MAKES SENSE FOR AN ACCESSIBILITY TOOL SPECIFICALLY
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An accessibility tool that only worked after signing in would be
useless to someone who needs it in order to sign in in the first place
- exactly the same logical problem UAC's secure desktop solves for
security (a prompt only a real user can answer, not something
already-running software could fake). Narrator being available at the
isolated sign-in session, independent of any user account, closes the
equivalent gap for accessibility: someone needing Narrator to navigate
the sign-in screen can access it before, not only after, authenticating.

WHY THIS WORKS AS AN ANSWER
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It explains what makes Narrator's sign-in-screen availability possible,
restates Chapter 4's own explanation of the secure desktop's isolation,
and draws the direct structural parallel between the two - both being
system-level sessions independent of any specific logged-in user - while
explaining why that same pattern specifically serves accessibility's own
practical need here.