I think this is an excellent idea, however I believe that the ideal place to answer questions in the specific "What is _?" format is the tag wikis.
For example, the question "What is SEO?" is answered pretty well by the seo tag wiki, "What is htaccess?" is answered in the htaccess tag wiki, "What is DNS" is answered in the dns tag wiki, etc.
It could be useful to get into the habit of linking to tag wikis when writing answers on this site, in cases where it would help to reference or substantiate the definitions of things. I know they often get forgotten about because they are buried so many clicks deep on the site, but you can find some good content by crawling the tag wiki catacombs.
I think that aside from "What is _" questions, there must be some other question formats that could make a good focus for this initiative and good addition to the site. I always seem to come back to What are the best ways to improve a site's position in Google when I am looking for an example of an exemplary, high-value community wiki post that gets linked to all the time.
To answer your points:
For open-ended "catch-all" questions that have long, many-part answers (such as the aforementioned site position question and some of the other ones Stephen linked to), I think the Community Wiki style Q&A can work great. Basically, marking a post Community Wiki means that the question and answers are no longer "owned" by specific users, but community-owned. Normally when you edit answers you want to edit them only for grammar while preserving the author's intent, but community wiki posts lift some of the rep limitations for editing, and allow multiple users to come together and add their own sections to create a true wiki entry, sort of like Wikipedia in some ways.
I think it would be fine if they are "semi-official" like some of the catch-all questions that we currently have. That is, it's understood that they are a different format than usual (where a user has a specific question that they want answered), yet they are still in the hands of the community as much as possible. I think that the real beauty of Stack Exchange is that things are mostly community-driven rather than being handed down as "official", and I feel like the general Stack Exchange Q&A tooling would work great for reviewing/moderating these types of questions. If I want to address a topic "officially" without having community members edit my content, correct me and disagree with me, I've got my personal blog for that :)
I do believe that the Q&A format would be ideal for this sort of content, rather than help-center-wiki style entries. The Q&A format (and to some extent the tag wiki format) is nice because it provides a standard interface that gives everyone the opportunity to write new answers and edit/update existing ones, and a voting system for the community to sort out which ones they think are the most useful.
On the mod side I believe they only let us edit a few of the help center pages anyways (like the on-topic page) - most of them are hard-coded. I'm pretty sure they don't let us create new help center pages either, but maybe the other mods can chime in about that. If you think about it, help center pages are in effect kind of like locked posts in that only mods are allowed to edit them; personally I like the idea of getting the community involved in stuff like this as much as possible.