Yes, Opera Dragonfly will still exist. The current beta version of Opera 15 includes the Web Inspector, but Opera is porting Opera Dragonfly to Blink. It will be on when ready.
Note. I was a formal product manager for Opera Dragonfly before switching companies.
Opera extensions use a different format for Blink-based Opera. It uses a subset of the Chrome extension model. There is a tool for converting existing extensions and documentation: http://dev.opera.com/extension-docs/
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