I'm started work with CJ Datafeed 1.5 plugin few days ago, and faced with same problems which you discussed here 1 year ago.
Since I found that up to now this functionality is not implemented in last plugin version 1.5, I modifyed the plugin code. The new added code supports adding new feed, updating earlier published with new metadata, and skipping it otherwise. In addition I implemented a simple support for large datafeed files processing, by adding "Start Line" to the Create post UI. It's automatically updated at plugin exit from Create post from datafeed, which allows simple to continue processing rest of the datafeed from next unprocessed feed. The updated plugin may be downloaded from