If France goes to war against the US then yes grain gets embargoed. In the long term the US could build a lot of modern warships not just monitors. It would not behove France to engage in an unwinnable war.Coastal monitors take ages to build. The first Monitor was the quickest; all the others took months longer than planned; it's eventually going to help, but that help is going to take at least six months to arrive. Even once done, the help is going to prevent attacks on US harbours, but a monitor is not a suitable instrument for breaking blockade.
As for grain imports, out of interest, are you picturing the US:
- passing an act to embargo grain exports to France.
- passing an act to embargo all grain exports.
- not passing a legal act.