Currently, the only cordova plugins for thumbnail creation are custom. Here are some of them: PhoneGap_Thumbnailer, cordova-thumbnailer-plugin. We haven't tested any of these, so I am not aware of how useful they can be in your scenario.
Concerning your second question – yes, it is possible to upload a file to Everlive through a third party service configured to use our REST API.
Please, do not hesitate to contact us if you have further questions.