Canonical will help to remove duplicate issues and also to consolidate your link values. I didn't see any issue with cross domain implementation.

If you add "noindex" to any of these pages, you won't get any link credit.

Noindex will take out the pages from the index of search engines. This will impact your traffic if your pages are already ranked.

As long as the category pages have different content, you will be fine. No need to remove them from the index unless you have specific requirement to do so.

For some reason, if you want only the first page to be ranked then go with the pagination hints which will consolidate your link value to the first page and most likely the first page will come up in the search result.

Note: Bing didn't officially endorse this practice and may be on its way.

http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html

Yes, they act as a 301 redirect and you will lose some link juice technically.

As long as you use them for Twitter, you will be fine. Anyway Twitter is not going to pass any link value to you.

For regular link building, use actual URL.

I agree with Ryan. As long as the old URLs are out there in the web, Google will follow them.

At the same time, redirected URL list will be very lengthy if you are maintaining a big site and maintaining them will consume your resources.

Check link value of the old URLs as a first step. If they have no value then you will be fine to retire such old URLs.

I would suggest keyword density not to exceed 2 to 3%.

Excessive usage will lead to keyword stuffing penalty.

Less usage may not relay the topic relevancy to search engine.

http://tagcrowd.com is a nice tool to visualize keyword concentration in any page.

In general Google results based on LSI - http://en.wikipedia.org/wiki/Latent_semantic_indexing

Unless someone optimized for the misspelled word, we will get the same result.

Noindex will take out the pages from the index of search engines. This will impact your traffic if your pages are already ranked.

As long as the category pages have different content, you will be fine. No need to remove them from the index unless you have specific requirement to do so.

For some reason, if you want only the first page to be ranked then go with the pagination hints which will consolidate your link value to the first page and most likely the first page will come up in the search result.

Note: Bing didn't officially endorse this practice and may be on its way.

http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html

Canonical will help to remove duplicate issues and also to consolidate your link values. I didn't see any issue with cross domain implementation.

If you add "noindex" to any of these pages, you won't get any link credit.