The downside to having the blog be on a separate domain is that your main site won't receive the direct benefit of the links, social shares etc that your content draws. While linking from the new blog might help pass some authority to the main site, I don't think it's as beneficial as having the content be on the main site.

But no, it shouldn't hurt anything. It just might not be worth the time when you could be continuing to add more content to your current blog instead of starting from scratch on a new one.

You'll have to decide if you think the benefit from having keyword optimized urls for your new articles is greater than the cost of the new content not benefitting the authority of the main site as much as it would if it was on the same domain (I hope that makes sense). Also, if your main site has any authority at all, your content on the current blog will probably rank higher than the same content would on a new blog. Optimized urls are not as weighty of a ranking factor as domain authority. So those are just a couple things to think about.

If you do decide to start the new separate blog, I'd avoid having lots of links going back and forth between the two domains. Keep them one-way, driving traffic and authority from the new blog to the main site.

Blog commenting is a crappy way to spend your time if your goal is to get quality backlinks for SEO purposes. With that said, I have clients that receive substantial (relevant) traffic on a daily basis from blog comments they posted a while ago, so i know first hand that blog comments can drive traffic.

Is it ever the MOST productive and efficient use of your time? Probably not. Not unless you're commenting on blogs you already read regularly. Going out of your way to search for relevant blogs, reading articles you have no interest in besides getting traffic from the comment link, and crafting comments that grab peoples attention and encourage click throughs...? I can only speak for myself, but that doesn't sound like the most effective use of my time.

I guess it depends on the length of the content. Typically, 15 occurrences is excessive, even if it alternated between the singular and plural versions. I would suggest developing your content around keyword themes using many variants, not just the singular and plural versions.

here's a blog post i did on the subject of keyword theming - http://searchsolve.blogspot.com/2012/10/developing-content-around-keyword.html 

I like irving's suggestion to see if the webmaster is willing to sell the site. whats the link profile like? any particular high authority links that might be giving it the advantage over your site?

I'm assuming the rel="bookmark" tag doesn't have any SEO implications but I just wanted to make sure it wasn't viewed like a nofollow by search engines.