...how beneficial will that really be if you don't use your keyword anchor text for each link?
Google's algorithms are looking at much more than the anchor text, so while it is optimal to have your keywords in the anchor text, it isn't essential. Don't forget the link is to your site and Google is going to crawl your site and decide for itself what words are relevant. But, honestly, I can't see a reason why you would ever need to create a bookmark that didn't have your keywords in it.
Obviously, I will use my keyword phrase as the title of the first link I submit to social bookmark sites. However, I cannot name each link to a new post the same thing.
Why not? There is no rule that bookmarks have to be unique. You could make every bookmark "keyword information" if you wanted to. They all point to different, yet relevant content and that is what matters.
But really, this shouldn't be a problem because what you're bookmarking is posts or pages and they have titles and hopefully you will find ways to work your keywords into the titles. And even if you can't, you can create the bookmark that way. For example, the title of your post is "Cool Stuff" but you could bookmark it as "Keyword phrase - Cool Stuff" In fact, when you're using the All In One SEO Pack, you should create titles exactly like that. When visitors come to your site, they will see the title as simply "Cool Stuff" but the Title tag in the html (which is what Google is reading) will say "Keyword phrase - Cool Stuff."
For question 2, check out this page:
Nofollow Link Social Media | SEO Training
Sorry, I can't help with #3.