EndNote Online (formerly Web) is an online back-up and collaboration option for EndNote. An excellent guide and tutorial on how to do this is available from Clarivate Analytics here.
Yes, you can, although there are different recommendations depending on your situation. The options are explained in this guide from Murdoch University.
Check out ProQuest’s EndNote guide page on Library syncing and sharing for brief tutorials and instructions on how to set up the Sync feature in EndNote Desktop and locate sync status information after selecting your Sync library.
Yes, you can. Your EndNote library can be attached to an email just like any other file. However, we recommend creating a compressed version of the library first (a file that includes your library and its corresponding data folder).