> Iran probably wasn't following the JCPOA

When are you talking about and what is that based on? It was the US that withdrew; the other countries continued. US intelligence concluded that Iran had no nuclear weapons program (or maybe, no active one; I don't recall).

> you'd also have to say you 100% trust American intelligence to verify

Nope, the plan was for the IEA to do their thing with on-the-ground inspectors, sensors, etc. As just one example, nuclear materials leak radiation and they can find indications of it in air, soil, etc., around faciltiies. Edit: Also, many other countries were involved and looking closely, including European powers and Israel.