I think except Ezine, you can use same article for other directories. As ezine don't accept duplicate content, you will have to submit a fresh article to ezine in order to get proper exposure to your website.

As far as I know, it is very important to have unique content whether it is site or any article as, there should not be any kind of repetition of content other wise it will not appear good.

In one word, content of an article should be different or else can be modified by targeting same keywords.

It depends on lots of things, but I would say 1 article a day per keyword should be more than enough. Don't need to overdo it, that's my opinion

Ezine Artical don't accept dupilicate content, you should submit a fresh artical in order to get a better page rank for your website and there should not be any kind of repition of content, but the keyword should be repeted twice.

Regards
shravan kumar

only two keywords are necessary for a article in resource box.

I always try to write as many as i can. I also avoid putting the same article on more than 1 directory so it's unique. I guess the amount of articles you need to do depends on your link building strategy and the competitiveness of the niche.

I don't use articles but recommend to spin them to submit to different directories

it depends on you keyword position, competition and need. well you can write one article per week.

it depand on your articles but by the google seo method you have to add 3.55% keyword in it