Welcome to the Q&A Forum

There is nothing wrong with having multiple sites / blogs on the same C block IP address. However, if you're trying to use your blogs to link to your products to boost SEO scores then you might want to consider other link building techniques in addition. Building backlinks from sites on same IP is okay, but you'll have greater benefits getting links from sites hosted on other servers.

Zeph, great advice on their heritage and translation not being literal. I did not think about this...

Plural is always better than singular. When it comes to singular term search your plural domain will still show for that keyword. IE if you search for "item" a domain with "items.com" will rank just as well as "item.com" because it already contains the singular version of that keyword.

You can confirm the above by performing a few searches and observing the results.

There are too many factors that influence pagerank and its hard to tell from just looking at your site without digging into it. Most commonly though your pagerank will not increase if your backlinks are coming from same PR site or lower PR sites. It also depends on whether your backlinks are follow or nofollow, you want follow links to increase PR.

Another factor that can be draining your PR faster than your backlinks are aiding it is follow/nofollow on your outgoing links. If you've got many or all outgoing links on follow then you're passing your PR out to those sites.

All of that being said, I wouldn't focus on PR so much. Many people think PR directly relates to how well your site does in terms of traffic but this simply isn't true.

I'm not sure if it can hurt you, however you can redirect their bad links to working pages with 301 redirects.

example: I link to your site with yousite.com/paeg.html (misstyped on purpose) but your page is actually located at yoursite.com/page.html

In this case create a 301 redirect to point from /paeg.html to /page.html and you will get people coming to the right place.