Joomla cannot know the link to a non core component like a sitemap. But this is not the root problem. Content with joomla isn't defined at all, only the default site.
Therefore you have to define every item for accesskeys in a sheme (accesskey (9), semantic tag (sitemap), link (com_xmap)), if there is no sitemap component installed, the sheme cannot work and joomla should not use it or should only use it limited.
With tab index, if you are going to set it explicitly, it would again be tied to the *percieved* importance of the site content. This will largely be determined by the site builder.
Right. I think this is bound on a special template or a template convention and not attached to the core.
It only makes sense to assign access keys based on module position
But the purpose of accesskeys is not to reach regions in a layout, accesskeys should reach default functions on a website.
