Nofollow all of the links between your sites and you can cross-promote all you want, safely. Link sitewide, in the footer wherever you like as long as they are nofollowed. .

If you are doing it for SEO though, I would probably not do that unless the sites are hosted in different places and ownership is not exposed. If they are completely separate you can one way link from the homepage (homepage only is best because it's always spidered and the homepage usually has the most PR) of the lesser sites to the main site.

It sounds to me though that this is for cross promotional and not for SEO so just nofollow them since that is completely safe. Just keep in mind that each place you nofollow a link to your other site you throw away a little bit of PR that would otherwise be passed to your internal pages.

Trellian global offers country specific keyword research information, but it's a paid service.

http://www.trellian.com/

It's totally not a problem for SEO.

Just weird from a user perspective when you tell people www.my "hyphen" site.com and type thru traffic.

I personally don't like hyphens for that reason, but i would rather a hyphen and having a much longer available domain name.

What are the two words, keywords or brand name?

Singular and plural search results vary and are still treated differently and if that's what you want to rank for (which was his question) i would recommend it in both the title tag and also include both singular and plural in the body content a couple times of course.

I definitely agree though it needs to be reader friendly from a marketing perspective, and also since title tag is limited space, doing keyword research might help you uncover a secondary keyword phrase that might be more valuable than including a plural or singular secondary keyword phrase.

Desk and Desks is good but wood twice is not   find strongest primary and secondary matches

Assuming the plural is more valuable than the singular, and wood is more valuable than wooden

<title>Buy Wood Desks | Wooden Desk Sets | Bob's Discount Furniture</title>

Assuming the singular is more valuable, and wooden is more valuable than wood

<title>Buy Wooden Desk Sets | Wood Desks | Bobs Discount Furniture</title>

you were penalized, so now your main pages don't rank for your main keywords, only your inner pages are ranking for keywords which aren't your main keywords. If you're penalized you won't rank for those anymore until you fix the issue so you're probably seeing the others you are ranking for surface.