You can check your backlinks from the Google webmaster tool, Majesticseo etc

3-4% keyword density is good for 400-500 words...

No there is no limit...

You can also use Yahoo, Bing for SEO...

Collect as many you can quality backlinks for your website, it will help you to achieve top ranking & maintaining it also...

Forum posting helps to get a quality backlinks but don't spam the forums with unrelated posts...

Use high pr social bookmarking & social networking sites..It will help to get high pr backlinks

Yes, create a HTML & XML sitemap for your website & then add your xml sitemap to the webmaster tool. It will help you to get crawl your website by the search engine

PR & ranking depends upon the backlinks but does not depend upon the quantity of the backlink, it is depend upon the quality of the backlinks

You can use facebook ads...Also you can create a fan page for your website & then promote that page

All off-page optimization techniques like directory submission, social bookmarking, article, press release submission, forum posting, blog posting etc