Zidoo AV out:
Thanks for the lead offer. I currently have a few things in the post: parts to make a custom lead; an HMDI to S-video + RCA converter that might get me a better result on old pre-HDMI projectors. I'll try them first and get back to you if I'm still stuck.

Mac connectivity:
I've found quite a few posts on other forums suggesting that Mac-Zidoo network comms are problematic. I don't want to repeat my previous posts, but just to to recap: I CAN see that the Zidoo and all Macs are present on the network via router Admin access; I CAN successfully ping the zidoo from any Mac on my network; I CAN now partially reach the Zidoo via SMB ('share' drive only - despite logging in with the Zidoo's SMB password); I CAN partially access the Zidoo from an iPad on the network via the app 'Zidoo Controller HD' (downloaded). I've had absolutely no joy trying to reach the Zidoo via http to do any app installs or other kind of management.

Zidoo Mac access:
Mac+Zidoo users might like to know there are two apps in the Mac app store: 'Zidoo Controller' and 'Zidoo Controller HD'. I have no idea how widely these are used: reviews are scant and unfavourable-to-dismissive.
I have found that Zidoo Controller HD does work on a iPad (only in landscape !!) and can speed up Home Theatre database tweaking by obviating the need to use the remote for text entry and other actions.
Unfortunately, none of the other functions whose icons appear in the ZCHD interface actually work on the iPad screen: they just activate the same options on the Zidoo display output that you'd get with the remote, anyway. That means that for app installation ZCHD dumps the user back with the http method (which won't work for me... etc...).
Like some of the software on the Zidoo itself, the apps do have the feel of an unfinished student project, but ZCHD does work a bit.

Is there any chance that the reconditioned Z9X Pro that I have has inherited a network access shortcoming from its previous brief outing? Should I flash it? Reset to Factory? Would either of those actions invalidate the guarantee?

THANKS... for reading my long post, and responding to the questions in a really informed way. I use a lot of video and audio tech devices, and it's pretty rare to find vendors with a deep "user's" knowledge of what they sell. On the basis of your response to my post, futeko are stand-out in that respect.

I purchased the Zidoo primarily for its comprehensive support of different movie file types (my 50-quid media player was starting to fall down on newer codecs, and the newer cheapos are starting to drop support of DVD menus). So far the Zidoo has opened and played every kind of file in my media library. I'm pleased about that.

I add a few comments, all about usability when you get deeper into the unit's supposed capabilities (beyond mere file playback). I absolutely understand that futeko is NOT responsible for Zidoo design decisions...

Q1: thanks for the user-guide links. Very useful - in all the ways that the Zidoo "manual" is not!

Q2. AV-out: I understand your point re. shortcomings of composite video signals. If well-amplified stereo is good enough for the user, the Zidoo AV-out presents itself as a possible audio output (by the time you reach the speaker, everything is analogue). Personally, I have a tolerably good pre-HDMI projector permanently installed (video in: composite; S-video; d-sub). Sometimes we like the big soft projection from that more than the crisp picture on the much smaller TV.
My question to the manufacturer is: if a function is deemed useful enough to incorporate into the product, why not give the user the info they need to use it properly? Have Zidoo just gone along with the trend to send everything via HDMI, 'cos all the thinking, parts-manufacture and coding on that has already been done?
The AV-out has the feel of a token offering: I'll try to figure it out myself, and solder up my own breakout lead.

Q3. Mac connectivity: specifically, I want to stream movies from the BFI Player app (on an iPad) via the Zidoo. Generally, I want to be able to administrate the Zidoo from a Mac or MacBook, since the UI of the device itself is pretty opaque, to me. I've been trying all possible ways to connect from several Macs.
 
I already enabled SMB on the Zidoo; and wireless; and I've tried direct ethernet from MacBook to Zidoo. I can log on to the premises router from any browser on any Mac, and see that the Zidoo and Macs are all present on the network. However, I cannot achieve log-on to the Zidoo via smb or http from any Mac (McBluna's Zidoo Web Frontend requires http log-on, doesn't it?). I don't have a non-Apple device that I could try.

AirPlay: I have a 7-year-old Roku stick that receives full-res video and audio from an iPad via AirPlay, and I expected the more sophisticated Zidoo to do it too. It's "good-but-not-good" to know that AirPlay is only partially implemented: it would be better, imo, if the manual or on-board menus stated those AirPlay (and Bluetooth) limitations, to save the user from wasting time trying to get full functionality from either. It seems especially weird not implement AirPlay fully on a media player that's all about movie files and movie viewing.

Q4 and Q5, HD collection and browsing: your answers are spot-on (actually, I figured them out myself after posting, by faffing, but kudos to you the deep knowledge).
Q4: That long-press on the remote OK button is another thing that might be good to have in the manual, or on-screen somewhere, I mean, EXPLICITLY. C'mon, guys and gals of Zidoo, let's not forget that all the lights have gone out on the user's remote and they can no longer see what any of the keys are called... even IF they know what they're supposed to be looking for...

Q5: Yep, every time the Zidoo does a refresh scan of the USB-connected HD the Home Theatre browsing display skips back to the start (of "list view", in my case). Stopping refresh scans does prevent that particular annoying behaviour, but then the poster images flicker all the time, presumably because they can't be "refreshed"... Hey Ho!

I've realised, just in time to save myself, that Home Theatre induces Stamp Collector Syndrome. It draws the user down the rabbit hole of getting their collection data absolutely "right", but only according to the law of the internet. It has too many niggly shortcomings for me, and luckily it's not the reason I got the Zidoo.

I'll probably put my media collection into 27 alphanumeric folders on the HD so that I can find things more quickly via the more basic Media Center app.

It would be great to work out the Mac connectivity thing, to poke around the possibilities of app installation, etc. Any suggestions gratefully received... I'll try them.
Apart from that, I'll avoid the more geeky aspects of the Zidoo UI and stick to the basics.

THANKS AGAIN for the exemplary customer service!