Keyword density is passe. Just make  sure you have used the targeted keywords in the title and a couples of times in the body content. But no need to use them as it is. For say if you are writing on Web design NY, you do not need to use the phrase as it.

I don't think a couple of links from the same domain is going to hurt you. If it becomes too many then it loses it natural value.

Sorry I have to disagree. If I am a contributing author of SEOmoz.org and I am getting 120 links for contributing 120 great posts in its blog, Google is not going to penalize me for sure. So, as long as the articles are great, and you are doing guest posts for traffic, you are safe. Rather I would go against the idea of guest posting just for the sake of link building.

to be on the safe side, it is better to get rid of them

No Idea as such but the thing is you have bunch of spammy links from low quality sites that you have built, it is better get rid of them. Since there is some possibilities that Google may have crawled those sites earlier and when then deindexed them finding them spammy, risk is always involved.

_First of all I would like to know why want to get your website bookmarked in social bookmarking sites. Unless there is a greater possibility of gaining some visitors, I do not think it is worth your time. Do not chase the algo, rather chase the visitors. _

yup .. search engine will see it as single page -

www.mywebsite.com/example-page.php

_Now, if you numerous pages with hash tags that means, you are cramming loads of content in the same page that is www.mywebsite.com/example-page.php. This may be bad for the users but SEO's perspective this URLs bad, really bad.

To target different pages, you need to have URLs without the hash tag. _