For anyone that’s been through Army basic training, would you take this watch?

I would buy this watch from the PX after basic for a graduation present :) Buy a couple black Casio like a W217H standard digital watches (less than 20 bucks) or the cheapest G-shock in black for basic. You don't want to be flashy in Basic.

What is the diffrence between an casio gw-9400-1er and a casio gw-9400-1cr

Looks like nothing. Casio only lists the 1er on their website. Typically the third set of numbers is the color way. The gw9400-1 are all black rangemans. Er versus cr are meant for different markets. Example gw9400-1jf is meant for the Japanese market but it's the exact same watch.

How accurate is the compass?

The compass is very accurate. I've compared it to a suunto mc-2 side by side and it's under 5 degree's off, IF at all. Casio's protrek and rangeman are accurate enough to use as long as they are calibrated.