I would agree, talking with the co-founders is the best route to go. Come up with good examples and/or comparisons for why you think the business model won't be a success but also propose a solution to what route you think the business should go.
Exactly. That's something you didn't mention in your story, and it is a crucial thing to do if you really believe that the product solves real problems. Don't let the unavoidable obstacles destroy your relationships. All of you are still learning (the brick&mortar business experience isn't that easy to transfer to tech startup) and that requires mistakes and dead-ends. You can treat your adventures so far as a lessons in Lean Startup fashion and check together if there's anything you all can improve.
Yes, I think after 15 months you should have a serious discussion with them. It might take a few days to agree on a plan. Make it clear you need income to go forward. They're probably thinking the same thing as you anyhow.
