not relevant ... my all mailboxes tab has gone missing

There is no way to find " all inboxes " or "all sent "

when I go to File > new viewer window

What told you that was the way to add to the Favorites section of the sidebar?

Follow the instructions to add to the Favorites that I linked above.

Also, if you hid the Favorites section, click the disclosure triangle at the right side of the Favorites header in the sidebar.

I’m not sure what tab you mean. I’m only aware of it showing up in Favorites. If you add it back, does it disappear again?

Use favorite mailboxes in Mail on Mac - Apple Support

There are no tabs in the sidebar in Mail. There are Mailboxes (folders), but no tabs.

but all sent does not show up

See the article in the link I posted, earlier.