In reply to Derek.

For instance, you cannot (as of now) stretch a header template with site title and navigation wide or full width on the top of the screen within a cover block.

Sure you can.
The “trick” is to add your Cover block, then within the cover add a Group block and toggle the ‘Inherit default layout’ on, then inside this group add another Group block (or Row variation or a Columns block if you prefer), and you should see the alignment options are available for this group block so set this to ‘Wide’ or ‘Full’. Then finally you can add your content to this inner group block.