By the way the way apple does a11y on iOS and OS X is impressive albeit they have had decades to perfect it. The idea of an accessibility editor mode is probably a good idea and rather on creating compromises, there is a special mode to create and alter blocks. This approach of "layering" accessibility is regularly practiced by OS makers and the iOS layer for a11y gives this access instantly to all apps that deploy on the app store.

An a11y layer in WordPress in general is probably what is needed rather than a break/fix for Gutenberg per se. And of course this is a huge project, but probably something that will help everyone including plugin and theme authors that want to bake in a11y into their product without having to reinvent the wheel.