Q1: If you have links to your website, your site will be in all of the data centers automatically. Google considers most site submissions to be spam, so you shouldn't submit your sites to search engines.
Q2: huh?
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Q2: The same way you would optimize any site. Even though its an article site it still has content and keywords and a title and can still be linked to. Its no different than optimizing any other site really.
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keyword density is how many time a specific keyword appears on your website compare to the total amount of words. So check out your keyword density and try to improve it. if you have firefox, right click and choose "keyword density" is a very easy way to do it. Otherwise, you have free SEO tools that allow you do do that.
It is an obsolete practice. It refers to the % of words on a web page that includes your Keyword.
Don't bother trying to understand it. Just make sure your keyword phrase is in your PT, PD, and Content... And for non Google SE's you can use the keyword meta tag.