1. Load is such a tough thing to put a number on. General rule of thumb is about 200lbs. However, does that mean a 12x12 cube of gold bars? No. When we test anything we have to test it at 5 times its actual weight due to "bouncing". Let common sense rule the day and if you do load up, try and prevent whatever it is from bouncing around. I don't ever see these failing and I'm guessing some people aren't always using common sense.
2. Again, depends on weight. I would recommend backer so that you're not just relying on the fastener, but also a "friction fit"from the backer. These are pretty thick root-cast bins that hold up really well...but if you just put a fastener through...whatever weight is on the shelf is all resting on that small fastener. I would be afraid of elongating that hole and it leaking.
Just my 2 cents. It seems like every time I respond, Halla posts some pictures of his solution on that same problem that has been working for him. So lets cross our fingers for more data