So I think the approach here will have to be - when isFlush:

• There should be no padding on the accordion item(s) so that the title text and chevron sit flush with the left and right edge of the border
• The focus/active/hover/etc. states should extend beyond the width of the accordion container

One way we can approach the focus/etc. states would be to use a pseudo element on the pseudo-class and give it something like

.cds--accordion__heading:focus::after {
  content: "";
  display: block;
  position: absolute;
  inline-size: calc(100% + $spacing-05 + $spacing-05);
  block-size: calc(); // this height will need calculated somehow based on 1rem (the default font size) + interior top/bottom padding or margin within the item
  inset-inline-start: $spacing-05;
  ... additional focus ring styling or whatever else
}

.cds--accordion__heading:hover::after {
  // ... hover styles
}

// ... etc states

This should ensure the overall accordion width is the same when isFlush and !isFlush, while allowing the states to visibly extend beyond the outside edges of the component.