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 caddys. As the caddys coupon includes your User ID, it cannot be resold or transferred in any way.

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

Yes. The old coupons from caddys will be kept on the page if it has not expired. New coupons will continuously be updated.