This isn't complicated. If a country is trying to develop nuclear weapons, the international community has three options:
- Get them to negotiate the end of their nuclear program
- Attempt to destroy it militarily
- Accept it and allow them to become a nuclear power
There really isn't anything else you can do. The JCPOA was an attempt at #1. You can argue that it was a bad deal, or insufficient, or wouldn't have worked, but regardless, once the US pulls out, we're implicitly admitting that option is off the table.
So that leaves us with either #2 or #3. Based on what happened with North Korea during the Bush years, my guess is that the Trump Administration will make a lot of threats about #2 but ultimately default to #3 (which is probably for the best, since military action is both hugely destabilizing and unlikely to achieve its goal).
That's why there's no Plan B. Because definitionally, it's almost impossible to come up with one.