Hello,
I must confess that I read the thread fairly quickly so I may have missed some points, but here we go with my observations.
Language
It has to be en-US since, at least for OOo, this is the default language and the one you get in addition to the localized version.
Though I would like to see colour, metre, centre written correctly I think you can live with this choice.
File format
My preference is to use ODF.
No doubt that the wiki version is much more dynamic and better suited for a collaborative environment, but it also has drawbacks (review of entries, printing, tracking of the program version)
Workflow
There is another thread on this so to keep it short, I think we should stay with something similar to OOoAuthors based on
draft --> review --> publish
What is missing in OOoAuthors is a definition of the role of publisher / "editor in chief" that is the person(s) that decide when to pull the trigger and publish. At present Jean takes care of this (and does a great job) but if it wasn't for her dedication I do not know how a document would be deemed ready for publishing.
So some work to do there.
Template
I guess it does not make much sense to depart from the template used by OOoAuthors that over time has been refined and improved.
You may want to consider making better use of colours, adding heading numbers, make it more modern-looking but I am not sure that should be a priority unless you want to give the LibO guides their own identity from the start.
Cheers,
Michele