There is no reflow necessary. For form submission, the issue is disorientation. This a good idea to improve usability. There are some authoring issues here in any case, and WCAG should include requirements related to inadvertent payments. This checkpoint should be part of Guideline 5.
ATs rely on the focus.
But I can't imagine a user agent that doesn't implement focus. Use wording consistent with checkpoint 5.8. Contact Greg Lowney and ask for a counter-proposal about minimal requirement. We need to prepare an agenda. Propose new checkpoints to see how it feels to harmonize the requirements related to comments in issue 348.