First of all welcome to IRC. You seem to have gone through the entire website and have lots of queries.
Let me clarify you one by one:
1. Cash credit for product posting(for the categories that have $ sign). - NO
2. Only reviews get cash credits. Reviews should be original (should not be copied from another website, if done so it will be deleted). You can add the review right in the review tab of the product.
3. Currently there is no review contest, but only blog contest; after writing your post, give us a link in the links section and also give it in the thread where the contest is posted. This will help us know who all are participating in the contest.
4. You can answer questions related to products in yahoo answers and other Q & A sites and give IRC links there. And give that URL link to IRC in the links section.
5. You can write posts about products listed in IRC in your blog or in knol and give the link here.
For all the cash credits detail, check the announcements tab. You will know.