The helmet may be "crunching" your sunglasses down on your nose because the lateral (around your head) adjustment is too loose. A general rule of thumb is to unfasten the chin strap and bend over. If the helmet falls off, the lateral adjustment should be tightened. I have also heard people complain that certain wraparound styles of sunglasses do not work with certain helmets. You may want to look at some cycling specific sunglasses (take your helmet with you when you go to purchase).

There are MTB helmets with visors, but they do not provide eye protection and probably won't give you the glare reduction your'e looking for. I've never seen a eye-shield type device on a cycling helmet and suspect that the aerodynamic effect might not be good.