I haven't used any other DTP software for a long time so I have no argument about what other applications call things. If "brush" (for how lines look) and "fill" (for how areas look) are the "industry standard" then I would be behind a decision to make that the way Scribus works. The less people have to re-learn when coming from other software the quicker they can get into it.
The icons in the Colours tab of the PP seem to already be that way but the rest of the UI doesn't really match up, e.g. the "brush" options are under the "Line" tab and they're also controlled by "Line Styles". It's confusing for beginners if there's both a line (tab) and a brush (icon) but they relate to the same thing. My point in trying to get a consensus on what they were called was to get consistency across the UI (and documentation). In other words, "brush" means the same thing everywhere and everyone knows what it means.
If everyone is happy with "brush" and "fill" then I'm happy too.
Your context-sensitive floating toolbar idea sounds very nice in theory. (I can only say "in theory" as I've not used it, since it doesn't exist yet.) I think it would certainly help people who are coming over from other software, such as word processors, as it's something like what they are more used to. (Now and again we get questions from beginners who have trouble with the concept of the Properties Palette.)
I'm not sure if I'm 100% sure about splitting some functionality between the PP and a toolbar. It might be better to keep it all in the same place (for consistency) but, then again, it might not. Having to go to one place to do some stuff but then another place to do other stuff related to the same thing might be a bit confusing. I've thought about this sort of thing myself in the past but have never come up with anything that I was really happy with. Maybe an expansion of your idea would lead to something really great, I just don't know.
This is one of the big problem areas with UI design: everyone's different and wants things differently. While one person might love using toolbars (or something similar), someone else will hate them (for whatever reason).
If I was being forced to make a decision about this, right now, I would probably come down of the side of just making the PP more usable. It's less of a controversial option and it's what existing users are used to. That's not to say that the toolbar option is "wrong" or anything, but it's probably just a bit of a step too far at this point. I think its one of those things that you've got to actually use in practice to see what you prefer and that's probably not practical (there could be a whole lot of work that may have to be scrapped if people don't like it).
Having said all of that, I can see the benefit of having some of the most used options readily available via a toolbar (font, size, fill colour, etc.). The only problem with that is: "Who says what the most used options are?" What I use most might not be what other people use most. This is a very tricky area that has no perfect solution. All you can do is try and come up with something that the majority of people are okay with.
I have the same issues as you when it comes to vertical space being used by the UI. Most displays now are wide screen, as you say, and I find that - especially when working on single-sided documents - I have a lot of unused space to the left and right. I normally fill these up with the Align and Layers palettes (because I use them a lot).
On the other hand, (there's always another hand) what about people who use their screen in a vertical orientation? A lot of DTP professionals - and others - use a vertically-oriented screen because that's the orientation of the pages they're working on. For these people, the horizontal space is more precious than vertical space. A big PP off to the side will fill up quite a bit of their display. I realise that most people are working on horizontal screens but it's worth keeping this in mind if Scribus is presenting itself as a product for professionals.
Basically what I'm saying is that what's right for some won't be right for others. If the UI can offer a solution that can be changed according to the user's individual requirements then that probably the best solution, but it's probably a lot more work.
As for primary and secondary panels, I agree that the user should be able to say which panels they want to be open. If they want everything open then that should be allowed. Conversely, if they only want "the basics" open then that should be allowed too. Let the user decide.
As for naming, I'm trying to get away from using "primary and secondary" and, probably, "parent and child" and, if possible, the words "panels" "sections" and others. It's difficult to describe but an analogy would be with cars. The driver has a "dashboard" which has most of the controls on it. Within the dashboard there is the "centre console" and the "instrument panel" and the "heater controls" and the "media centre", etc. These things all have different names and it would be nice to be able to give each of the things in the PP a specific name so we don't have to use sentences like "open the PP Text tab, then expand the Columns & Text Distances sub-panel and press the Tabulators button". We could just say "go to the <thingy>", whatever the <thingy> was called, and it would be obvious where that was.
This might be a very difficult thing to do, and it could be completely pointless in the end, but I think it's worth thinking about as it might give some insight as to a better configuration for the PP. Then again, it might not, but it's worth a try.