This isn't a fix, but a sort of a work around.
If you quit your safari technology preview app, then head to `/Users/(your user)/Library/Preferences/` then there should be a file `com.apple.SafariTechnologyPreview.plist` Delete that. then open Safari Technology Preview again. Your dev toolbar should be on the bottom again.
I haven't tested if this will fix the issue of the missing tabs, but I now have tabs again. And can undock the tools and use them in full screen mode side by side to get a similar effect.
Ok, so This is a one time thing, I tested to see if it would do it again, and I was stuck, so it seems not to be a good idea to delete the plist file. Though I haven't seen any side effects from it.
But a better work around is using cmd+shift+d. This will toggle the web instpector from the current location to the last location. I happened upon this one because that is the shortcut in chrome and safari apperently has adopted it.