You have made a thoughtful if theoretical argument. I suspect that most people are not used to reading Futura. And that lack of familiarity will make it harder for them not take in what is being written or reduce their speed in a meaningful way - their ability to make use of it will be probably be less.

http://www.apa.org/monitor/2010/11/read-onscreen.aspx

Still, it might be worth posting the kind of layout you intend. I can imagine this sort of thing up to a point but seeing it is likely to encourage a more substantial and more detailed response.

But leaving all of this aside the thing that strikes me about your task is that you need an unusual amount of visual distinction to be do this well. I would describe this as a need for semantic or role distinction within the text. I get this idea from the paragraph below.

" Within the simple structure of short frames, the text is difficult to set. It contains a mix of regular text, acronyms, titles of works, titles that include acronyms, code to look at, code to type, code quoted within the text itself (including alpha-numeric variable names), the names of keys off a keyboard (including escape and control sequences), formal terms from the system model, answers to be covered up until the questions have been solved, answers that include code, and so on."

Given that this is the case the question seems to me - what combination of layout and font style options will give you sufficient support for all these distinctions? If that is the primary question I don't think Futura is a defendable option. I don't think It will give you enough distinctive styles.

Some thoughts here:
http://typophile.com/node/75913

A personal experience: years ago a read a +400 page book - a great book - entilrely set in Futura. The book, "Can't Stop Won't Stop : A History of the Hip Hop Generation", is written in English (not my language) and I remember started reading it on a plane coming back from Indonesia - just to say that there were some objective obstacles. And you know what? I had no problems in reading it at all; as far as I remember the text could have been set a little bigger, but I ran through the book easily. Here it is, by the way:

http://www.amazon.com/Cant-Stop-Wont-History-Generation/dp/031230143X

...most people are not used to reading Futura.

How can that be, when it has been one of the best-selling, most used typefaces for eighty years?

I agree that Futura is problematic for body text. But that doesn't mean that one should exaggerate the extent of the problem. I would rate it as only slightly worse than, say, Helvetica. (Univers is significantly better, although still only by a small amount, but a small but definite amount, than Helvetica for body text, although to my eyes it is a less attractive face.)

Even an inferior typeface based on Futura for use with crude proportional-spacing typewriters, Mid-Century, was used for many manuals for Xerox and Digitial Equipment Corporation computers (instead of a more conventional serif Roman like Documentary).

If you want the highest possible readability, therefore, I would not recommend Futura. But the degree to which readability is compromised is so slight in practice that if Futura suits the look or the mood you wish to create, I see no reason not to go for it. I would even say the same about Kabel.

Do it, if you want a time consuming and hard work, use Futura for extended text settings, otherwise use a different typeface. I had to struggle with with Futura LT (which is not Neuville's interpretation, of course) for more than a year recently … brochures, thick technical catalogs, inhouse formulars, ads, branding stuff etc.

I'm going to post some samples in the week ahead, you can see how I'm trying to use it.

If Futura can be made to work as a body face, as Bringhurst posits, I'd hope this is the kind of project for which it would work - it's computer science, medical software, neutral, techie, engineering, and intended to be read in short bites - so I'd like to wrestle with it publicly a bit to see what we can do with it.

If, in the end, we can't make it work, I'll switch to something else. I'm researching my backup options now in case we have to abandon Futura for this project.

When picking a typeface for a text, it is often wise to stop a minute and look at your motives. Are you picking this typeface because it is the best way to present the text? Or are other motives involved "See, I can do it," or "No one has ever done this before." Sometimes motives are more pedestrian, as with "I'm so tired of the typeface that would be best for this text."

Most of us with any spirit fall prey to this from time to time. I've heard any number of designers give elaborate theoretical arguments why their choice was perfect for the piece. Except in rare instances, they've been wrong. (And truth to tell, I've done this too.)

So the question comes up, are you posting here to get a bit of validation? That would seem to indicate you've got some doubts.

I'd turn the question around. Can Futura be made to work? Probably. Could another typeface be chosen that would work as well with less effort by both the typesetter (letterfit, kerning, spacing) and the reader? If so, at least you know your motives. Would another typeface work better? Now that makes it hard -- your motives are in conflict with the best interests of project.

If either of the last two are true, the typeface that is chosen is the result of who has final say, rather than the text and the reader. Happens all the time.

I would use a Futura with shortened as/descenders, because that's the problem area, in my opinion. Your proposed setting (12 on 18) is not very efficient and too ‘blonde’.