Hello there!
I’m the author of NGFB Open Graph+, and would have been happy to give you a hand with this in the wordpress.org support forum: http://wordpress.org/support/plugin/nextgen-facebook
A few things come to mind…
– There are no conflicts between NGFB Open Graph+ and WordPress SEO. NGFB specifically tests for other (known) SEO plugins, and issues warning to turn off certain options.
– Unless configured with a large enough default image, NGFB will not add images to index-type webpages unless the first post in that index (or in the search results), has a suitable image.
– Once a URL has been shared on Facebook, you can no longer modify / refresh it’s cache. The errors you are seeing from the Facebook Debugger may have come from its own cache elements, and not based on your current page headers.
– I see you are no longer adding Open Graph meta tags. You may want to reconsider if you care about social engagement. 😉
js.