Normally, the military discounts programs on some occasions of the year. In order to know whether it is offered or not is depended on Namecheap's policy sales, you can visit the store now for your reference.

In case you enter a discount code during checkout that doesn't exist or isn't currently active, then you can see the message: “Unable to find a valid discount matching the code entered”.

Of course not, most coupons may be used only by you when you received them directly from Namecheap. As the Namecheap coupon includes your User ID, it cannot be resold or transferred in any way.

Usually, Coupon Stacking is not allowed at Namecheap. However, the store will surely give out notice if available.