It is my understanding that the Marines also had these weapons. They went over the side about two weeks after a landing as I recall.
Not to my knowledge. There was the 663rd Field Artillery Battalion deployed to Okinawa from 1955 to 1957. But this was Army not Marines.
The Marines had other kinds of nuclear like W33 and W48 shells fired from M110 and M114 howitzers.
marines never had them. based on information gained from examining the 2 german 28cm K-5 railroad guns captured after the anzio breakout (finally). both were brought to the states and used to fix one to test fire. K-5 LEOPOLD used to sit across the street from the atomic cannon at APG