Google knows the original by using the rel=canonical tag. These work across domains as well. If you syndicate content, it doesn't even need to link back to you, nofollow or dofollow. All you need to do is have the rel=canonical tag to the original post and you're good.

I would probably not include all 100 customers. Maybe a sampling of your customer base?

You could also nofollow the external links to sites that could be questionable. You could also noindex, nofollow that page as well. All are options.

In fact, you could just put 100 links on the page if you want. I highly doubt you will incur any kind of penalty at all. Matt Cutts has dispelled the myth that you can only have 100 links on a page. You can have as many as you want as long as it makes sense and doesn't look like your are spamming search engines with link farms. I probably wouldn't worry too much about it.

That is an awful directory. None of the entries in the categories I looked at were even close, they were all over the place. Google has figured this out I would guess because of a very high number of reciprocal links. Most spammy directories require a reciprocal link to be included, so they likely have several hundred or even thousands of domains linking to them. Google would naturally penalize this. OSE doesn't seem to have a way to categorize reciprocal links, but that would be very incredible if they could.

Imagine being able to see your followed links, nofollowed links, and right in there with it, your reciprocal links. Then they could reduce the authority of sites with over 90% reciprocal links or something. Would be good for an SEO to know as well especially when taking over a website you haven't done work on before. That's good info.

You only need to use a sitemap index file if you have over 50,000 URL's. So if your site is that massive, then yes you'll want to do this. Basically all you do is create a bunch of sitemaps. You can then create a sitemap index file that is just a list of links to those sitemaps. You then submit your sitemap index file to google or each sitemap individually. Just glancing at your link it is pretty darn clear, there's not much to it.

I use xml-sitemaps.com if the website has proper navigation and is less than 500 pages. It works pretty well, you can download the sitemap afterwards in xml, html or txt.