Your ad placement looks fine. It's already fine to me as it is, you could change some things for testing, but at the moment it's already fine, so test only if you're not happy with your CTR. You can remove the "Posted by admin" bit. Also you can remove the two blocks: Who's new & Who's online You should add something more than the RSS icon in the slidedown region of marinelli theme, or don't display it at all, better use it though it's nice.
ps how do you remove the posted by admin part!?!?! (thanks in advance!).... and thank you for the review!
Placement of your ads look good to me, Although I'm not to fond of the 3 column layout, it seems to cluttered to me. Just my personal opinion.
It will depend on the theme. Sometimes you need to edit the single.php file. Alternatively, you could just change the name to something more meaningful than "admin" and keep it. That would be in the user settings, I believe it's called "display publicly as." I'm also not wild about the current three-column layout, especially because the main content area is so narrow.
To remove the "posted by" and "date" info goto admininister -> themes -> configure (tab). Scroll down to the section labeled 'Display post information on' and uncheck the content types under it that you do not want the information displayed for.