Senior co-author is jeopardizing two years of work
I suggest that you get all of the co-authors together to have a discussion. When you organise the meeting, say something neutral like "let's get together to figure out what to with this paper".

If your senior co-author has a good point about the methodology being flawed, then the other co-authors may suggest a way to salvage the paper. They also have an incentive to get the paper published, if possible.

If the other co-authors don't agree with the senior co-author, then they can put forward their arguments, and hopefully a resolution will be reached.