I see the value of associating anchor text with content links rather than navigation, but I agree with EGOL more than Ryan on this one. What if users are viewing the site with CSS disabled? Maybe because it's easier to view with a screen reader for blind/partially sighted people - it's not very user-friendly having the main navigation links only at the bottom is it?

I agree that it's better to add it to your own site. What if people like your blog and link to it, but it's not on your site? You've lost some link juice there. You can submit it to blog directories too - more direct links to your site. For ecommerce sites there's a lot of content duplication with other sites when the same products are sold, so like you say in your last sentence, a blog adds regular unique content.

I don't believe you'll get 'penalised by Google' if the title tag is too long - as you know your full title just won't display in the search engine results pages (SERPs). If your site name is well-known and/or might improve click-throughs it is worth having it visible in the title tag. Personally I don't see a problem with the site name cutting off on some occasions - it's not going to stop people clicking is it...?

The meta description should be geared more towards getting click-throughs - as you probably know the words people search for show bold on the SERPs so as long as it reads naturally and doesn't look spammy, it's worth having other keywords in there too. Just remember it's there to encourage people to click to your site.

You can optimise titles for both. It's best to have your prime keyphrase near the beginning of the title - and make sure the title is not too long.

The way you suggested does make it look like you're just stuffing the keywords in for the sake of it though...again it depends on your keywords but something like this would look more natural with the downside of not having one of the phrases nearer the start:

"Green Apple seller: the best Green Apples"

Make sure the language you use is natural. You might be able to rule out one or the other if it wouldn't be natural in your content.

Advice I've read in the past has recommended to optimise for both, though it depends on the term of course; some might be better as plural, some might not. Compare the search results for each and if one is more competitive than the other. Each might bring up completely different results, I think one example I read was that the singular showed e-commerce sites whereas the plural showed descriptive sites - I can't remember the example that was used.

Optimising for the plural is more likely to help with the singular than the other way around, but I think Google can usually tell they're related e.g. www.google.co.uk/search?q=red+bull+sticker - in more cases a longer tail keyphrase will probably show for both plural and singular, even if you just optimise for one of them.