it works, but as the description says, you need 1 point to use, before you get the 999. When you start a career, with no Tutorial you start with 0 points. So need to do a mission, or do tutorial and you'll have 20ish after that. then use trainer.
about unlimited deckgun ammo, there can be a problem where the gunners stop shooting, if your not shoting em yourself. As if the gunners think you are outta ammo, all you need to do it mount the gun and your gun ammo should be maxed again.
Not sure if this last part is a "bug"ish thing, or... does the trainer option for Ammo only work when mounted on gun? cuz theres no way too see how much ammo you got, unless you mount the gun right?