Its confusing how you have to look it up. You restart game, start going through it, and then you look at your menu all where you keep all the other logs and stuff.
The original infestation was never destroyed, it was just repressed. As soon as they moved the marker off the amplifier, it allowed the necro's from the original government infestation to be active again.
A Merchant Marine isn't like a Devil Dog USMC member. They dont storm beaches and all that stuff, they are just a reserve fleet that can be called to duty during war time, and the mariners on board end up with the title marine.
It would explain why he would at least be comfortable with the assault rifle though. While he probably didn't go around storming other ships and the like, they probably showed him how to use one in case they were ever boarded.
Merchant Marines are basically just badass sailors. We haven't used a huge number since WWII, but they were HUGE for us in WWII. And knowing how things cycle back and forth, can and probably will be used again. (Some Merchant Marine guys still get around under Federal service today).