I've been reading through this very long thread, and I think you're the only who's not getting lost in details that can be figured out later but still focusses on the basics
That's not meant as an offence to anyone, since those details need to be considered later, and a discussion on them is absolutely necessary.
I've also seen that you already made a lot of progress at https://github.com/nitramr/scribus-indigo/wiki
. Well done!
Now that Scribus 1.5.3 is ready for relase, maybe we can get back to the basics and see how we can continue from there.
In my opinion the top priority must be several issues before 1.5.4 can be released (or before your branch can be integrated into svn):
1) Rip both properties palettes apart and modularise them (and make them dockable for the time being).
2) Make the modularised palette elements content-dependent, which means, for example, that no image-frame-related element is visible when a text frame has been selected.
3) Enable access to the relevant settings dialogues via the modularised elements, i.e., to Styles for text frames, lines and tables, to Colours & Fills for general frame settings and shapes.
4) Introduce frame styles.
5) Introduce a feature to save the current workspace, both with a document and as an external file. These options aren't mutually exclusive.
Points 3 to 5 are among the most requested features I heard from professionals.
The rest will be sorted out and optimised via real world testing: trial and error
I absolutely adore your drafts for horizontally layouted palettes, and I'm convinced that this is the way to go, especially for text. I think, though, that for text a dialogue needs at least 3 tabs on top of the dialogue to provide access to all features. I also think that those horizontally designed dialogues need to be able to be moved around on canvas or docked at the top or bottom. Having them fixed at the top might cause unforeseen problems, so it's better to allow for some flexibility.
And here's another suggestion: InDesign provides access to advanced features like the OpenType ones via a menu. Perhaps it would make sense to add a new menu "Typography" or "Advanced"?
One nitpick: You should use Qt Designer to see what's realistically possible with Qt and how it will all look at the end on multiple platforms.