when built in factory it doesn't autobuild, you can tell if the build button is outlined for whether it is autobuilding or not, right click to activate/deactivate autobuild, left click to tell it to build only 1 nuke (per click)
to answer previous questions, it can build while moving (like acu TML), you can cancel it building a nuke with stop order (it will lose all progress like nuke launcher would when you do) or pause it with pause button
for some reason it uses same bar for its long range missile that it fires for free and for building the nuke progress, idk if it is possible to separate the 2 bars in code (similar problem exists for battleships like summit class, where the 1 bar shows the turret's reloads, but each turret reloads independently and there are 3 of them on the ship)