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meta-keywords and description don't count towards the content on your website, based on whatever I've read, but the description is extremely valuable in getting people to click.  Alt tags do have some influence, but make sure the alt tag is for the user, and not just for the search engines.  Here's what SeoMoz says about description tags: http://www.seomoz.org/learn-seo/meta-description

I would definitely go for the contextual link from the most relevant page (I assume that this is a high quality page, and that it is reasonably well linked within the website). I believe not only that google will put more weight on a link like this, but it will also send you targeted visitors which should be easier to convert.

Forget this.... " What should I do to increase my PA & DA and to have top 10 positions by other keywords?" Do a lot more of this.... "Now I`m adding new pages with unique content."

People may have been tweeting about only some of the pages, or linked to some of the pages, or maybe the bot just randomly decided to index some of them and not others?

The amount of link juice you will receive is the same in any of those examples. Your question is similar to "which weighs more, a pound of feathers or a pound of lead". A link's value is determined primarily by the linking page's DA/PA/ or PR (however you measure link value) and the number of links on the page. There are other factors such as a link in the content area of a page offer more value then links in footer areas for example. Based on your question it seems the link will appear in the content area of your page. We know Google associates anchor text to a link's value, but I am not aware of that text influencing the amount of value that is passed.