prefers-reduced-motion: reduce leaves loading spinner and skeleton shimmer undefined — remove or simplify?

Undecided behavior in a reduced-motion workflow

I'm applying the prefers-reduced-motion media query (Media Queries Level 5) to a user-facing checkout flow to address vestibular-disorder risk, in line with WCAG 2.1 SC 2.3.3 and 2.2.2. The large spatial effects are easy calls: parallax sections, full-screen slide transitions, and zoom-on-scroll get replaced with opacity fades inside @media (prefers-reduced-motion: reduce).

The unresolved part is the gray zone the spec leaves to author judgment. The flow contains a loading spinner on the payment submit button, a skeleton shimmer while cart totals recalculate, and a scroll-driven reveal on the order summary. None of the guidance I can find normatively defines "essential" motion, and sources disagree on whether these should be kept in simplified form or removed entirely. I'm also wary of the opposite failure: stripping the spinner outright removes the only busy-state feedback, which could create a new accessibility problem.

Assume modern evergreen browsers; the JavaScript-driven pieces already read matchMedia('(prefers-reduced-motion: reduce)').

1. Is there any accepted criterion for classifying motion like spinners, shimmers, and scroll reveals as essential vs. non-essential under reduce?
2. For state-communicating animation, is replacing movement with a static or opacity-only indicator considered sufficient for SC 2.3.3?
3. Should the reduce branch shorten durations, or is removal of spatial movement the safer interpretation?