Hi Mark,
I see your questions have been sitting unaswered for a while now. Let me take a crack at it.
1. Keyword Difficulty - "...my current thinking is merely to ... make sure the keyword is relevant to the article as the first priority and use the tool to check out search volume." Good strategy. When it comes to blogging, one of the main objectives is to built relevance and trust. Picking the most relevant, most applicable phrases that match your intended content is what counts. Volume should only be a consideration if you have options that are equally relevant.
"Not sure what I should read into a zero for recent Bing searches. Is that really an important factor? **"**You are correct. Google doesn't give Moz access to it's keyword data. I would again use relevance as the deciding criteria and not worry about volume.
2. Titles - I think your question is "should I use the keyword phrase in my title and if so, does it have to be exact match". The answer is yes and no. Yes you should use your chosen keyword phrase in your title tag. It doesn't have be an exact match but understand that if you're in a highly competitive market (and certainly the wedding market is competitive), then the closer you are to an exact match the better. Again here though, your first priority should be in appealing to searchers. So if you think "Involving children in our wedding" will be compelling enough to attract clicks thru to your site then go for it. If you think something like "The pros and cons of involving children in our wedding" (or something else) would be more compelling, I'd go for that.
3. URL - No reason for them not to be the same other than length. Google ignores stop words like "and" and "the". You can shorten your URL by removing them and still achieve your objectives.
4. Meta description - yep. Make it human friendly and use your keyword phrase. Also make it compelling and between 150-160 characters. If you and 9 other articles all contain the same title and description tags, why should I click on yours?
5. "Make sure that the theme (we are moving to WordPress) uses H1 for the page header/title and H2 for sections within the document". Here you just want to make sure when you're publishing your content that you don't end up with multiple H1s. I see that a lot with different Wordpress themes. Some allow you to toggle off and on using the page or post title (at the top) as your H1. You'd have to try it and see.
6. 301 redirects - yes, redirect the old to the new. Any you want to retire, redirect them to their closest relative. If there is nothing similar to redirect retired pages to, consider setting up a custom page-not-found (404) page that explains the URL the visitor is searching for no longer exists and point them to your most popular pages, your blog, a human-readable sitemap, or whatever else makes sense. Don't just let it fall thru, unattended.
"Anything leaping out as missing/problematic?"
If you're new to wordpress, you need to be aware that it creates a lot of duplicate content as a result of how content is stored so as it be easily accessible to readers. The Wordpress for SEO by Yoast plugin is the one I use to manage SEO aspects of the site. I recommend it. It's not completely plug-and-play. You'll need to configure the back end to minimize duplication.
The Yoast plugin also allows you to enter "focus keywords", phrases you think might be good to optimize your page or post for. As I said earlier, I use Keyword Difficulty for main site pages, but just the Yoast plugin and its build-in integration with Google suggest for finding article keywords. You might consider just using it.
Lastly, Moz has best practice guidelines for crafting all these parts and pieces. It would be worth your while to review them if you have not done so already. Sounds like you might already have though.
Good luck!