That depends on what percentage of your links are made up using those keywords. Negative SEO usually takes a LOT more effort than building a few crappy links to your website. The same has been done to some of my client sites in the past and it's not made any difference to their SERPs

Anyway, the simplest solution here is to login to Google Webmaster tools and use the Disavow Links tool so that at least they won't count towards your backlink profile.

http://support.google.com/webmasters/bin/answer.py?hl=en&answer=2648487

Hope this helps.

As meta keywords - no. As part of your meta description - maybe. Like with your body text, it must flow naturally.

Your meta description should contain your main keyphrases, by which time you wont have much room left for anything else as Google only looks at the first 150 - 160 characters anyway. Anything beyond that a) will get truncated and b) looks like keyword stuffing.

In your case, I'd focus on sliding the LSI keywords in to your body content where natural and not worry about including them in your description.

p.s It should go without saying that 'meta keywords' holds pretty much zero value in SEO anymore. Even Yahoo doesnt care about them much these days.

Further reading: http://www.seomoz.org/learn-seo/meta-description