I have multiple tabs in multiple panes, but when I've move from my widescreen to my laptop, the panes no longer works well at half-width.
Can I keep all the existing files open, but move/merge/combine them into a single pane? (Without having to drag each of the tabs individually?)
Yes, according to Editor Basics:
To unsplit the screen, from the context menu, select
Unsplit
orUnsplit All
to unsplit all the split frames.
Go to Window
> Editor Tabs
> Unsplit