By selecting Adwaita dark for “Legacy applications” in the Tweaks app.
Not sure how to answer… We all know about the various workarounds we have to use with the L5 to achieve “right click” context menus, and copy/paste sometimes. 
But it could be “too much functionality” for GNOME developers’ “philosophy.”
FTFY 
I know newer versions of Nautilus, so I know that right click, double click and more are fixed beyond Crimson. GNOME wants to support phones completely, so at least at some point I do not see any issue here. And if a file manager has fully touch and convergence support while another has not, but the split screen, I would probably stick without split screen and mimimi about the missing feature. 
nautilus has that too. Using tabs. You can have as many tabs open as you want (much like a web browser). And I do frequently drag and drop between tabs. However I have no idea whether, in terms of screen real estate etc., this approach would be usable on the Librem 5.
Files (Nautilus) has it not. Tabs are something completely different! And Dolphin supports tabs and split view btw. The benefit of split view is to see both folders at the same time. But at least drag and drop is functional via tab in later versions (not as fast, but still).
