peer review - How do I address my research publication being 'scooped'?

Back in spring 2016, I submitted a manuscript for publication. I revised the manuscript in winter 2016 and have been waiting for reviews since then. Two of the three reviewers responded in a timely manner, while the third has consistently delayed their response. This delay has been going on for almost half a year. I have been patiently waiting and in communication with the editor. The editor has sent numerous reminders and personal emails to the third reviewer to send their re-review, but nothing has come out of these efforts. The editor recently sent out the manuscript to a new 3rd reviewer who is considered more ‘reliable’ but the deadline has passed.

During this time, I received an email to review a new manuscript submitted to a large journal with similar research, methods and analyses used. I feel that this other manuscript will scoop my research that has been sitting for nearly a year and been delayed continuously.

As an aside, in Winter 2016 I saw a collaborator present research similar to mine (in terms of methodology used). This methodology has not been applied in my field until I put together my manuscript. I was surprised to see this in their presentation at a conference and was confused by how they knew what I was working on (it didn’t share with them). This is speculative, but my best guess (and this is also based on this person’s track record with others) is that they had access to my manuscript that was under review. The new manuscript that surfaced this past week doesn't include this person as a coauthor, but it seems likely that they encouraged these authors to use methods and research similar to mine.

For me personally, I want to see the manuscript published with an acceptance date that precedes this new manuscript. I notified the editor and they alerted the chief editor, but nothing has been done yet to rectify the situation. Typically, two reviews are required for this journal, but a third volunteered, and as a result, my manuscript requires three reviewers and not two. The editor and chief editor acknowledge the seriousness of the issue, but I don’t know if/how to encourage them to take further action rather than continuing to wait for a response to the third reviewer. I feel like this approach has been overused in my circumstance and has not produced anything out of it. I would like to move forward with the response from the two reviewers, but I don’t know if that is possible or how to suggest this to the chief editor.

Is there anything else that you recommend I do to rectify this situation?