RE: Fletching cement
In my experience its about like canned milk... it'll keep forever.... until you open it... then its a ticking bomb. Glue can actually go bad after a period of time (as you have expereinced). In a best case scenario it will turn to jelly.. worst case it won't... but the chemical properties are not what they should be.. and while it will stick and look fine.... as soon as you shoot it or put some side pressure on it the vane will come flying off like confetti out of a party popper.
I always buy mine in small quantities... usually just about enough for whatever I need. I use AAE Quickset gel for the actual fletching and I anchor the front and back with Goat-tuff.