Silo Structure - Let's get it right...

As far as I know, it is not mandatory to link every child page to every child page within a silo. Honestly that can look kinda spammy. Obviously you will have certain child pages within a silo that you want to rank for also, so I simply pay more attention to linking 'said' child page with as many other child pages within the silo where it makes sense. I generally will have four or five links on a child page to other child pages within a silo of 10 or more child pages.

As for closing off the silo, all your child pages should link back to the landing page anyway so as long as your final child page does this, then the silo is not leaking any pr.

The thing is that I don't really have a specific page I want to rank for. I might point a high PR link to the most competitive term, and then link from this page to B, B to C, C to D etc. so I can rank the other pages.

I was thinking about closing off the silo (pointing the last link to the most competitive term), and then using another high PR link to point to more posts in the same category, rinse and repeat.

I thought it might have been a good to link to one other page in the body of text, and then have links in the sidebar or at the end of post (as they carry less weight), linking to other pages in the same category, just so a user will never be more than a click or two away to other posts in the category.

I did not advise against linking child pages to child pages, I said that it looks spammy to link all of your child pages to all of your child pages.
If you have a silo with 10 supporting child pages, that would mean you have ten internal pages on any one child page, or a massive sidebar which looks horrible which is why I suggested you link to a few, mainly the ones you want to rank.
You could mix it up, making sure that all of your child pages get links from a few other child pages to pass on link juice.

If you already have a site with ranked pages you need to be extremely careful trying to overhaul the entire sites link structure. Google can be touchy about not being able to find links that once existed on a previous googlebot crawl. I would start with a small site or maybe even a single category & do your tweaking until you get everything setup like you want.

I'm not sure what your talking about by closing the silo, a simple category based breadcrumb nav. link will take care of linking back to the category page If that's what your trying to do without it looking obvious on the page.

I don't want to make this complicated but something else you can do to help manage a lot of links in a single category is to use the Wordpress Tags for organizing small groups of pages in a single category. If you use the WP Tags for organizing category pages I would make it a point to completely remove anything else in your theme that deals with WP Tags.

You don't have to have a WP Tag template page in your WP theme to be able to use WP Tags for organizing other pages on the site/blog.

Example, say you had 30 pages in a single category, you could WP-Tag those 30 pages in 3 groups of pages so that each page in those 3 individual groups only shows links on those pages for the other 9 pages in the group & so on for the other two groups of WP-tagged pages in the same category.

On your 10 web pages in one group you would display the links based on the WP-Tag (keyword) for that one group of pages, those pages are still in the same category as the other two WP-tags.

Using WP Tags for organizing pages isn't the same as a sub-domain, a sub-domain will nest deeper in your category creating longer URLs. Personally I don't like using sub-domains, it just makes things more complicated the deeper the URL is nested.

I hope I described the WP Tags so it's easy to understand, really it's a simple way to manage a lot of pages/links.