When I have many tab in a tab group and there are too many then I have to scroll left/right. It would be better if I could have the tab group display across the lister and viewer panes. I looked but couldn't find a setting for this. Am I missing it? How can I accomplish this view?
No way to do that. The tabs are only under / over / left or right of the file display they are part of.
In particular, you can have two file displays side by side in one window, with separate tabs in each, so having the tab bar go across the whole window would be a problem there.
Some people who tend to have lots of tabs open prefer to have them on the left or right, since that gives you space for more to be visible at once.
I realize they are two panes, and maybe three if you have dual pane enabled, but I was hoping that maybe it could be treated like a tool bar. The side bars take up too much horizontal space. But it is what it is.
You could have a toolbar of buttons that navigate to different folders, and use those a bit like tabs. (More like favorites than tabs, since you'll lose any selection etc. when changing folders that way.)