I believe you are correct that using epoxy adhesive on materials with different coefficients of expansion causes a sheer problem for the joint.
The only epoxy joint failure I’ve had was in joining a levered clamp with a steel base to the plywood surface of a telescope platform. On the first cold viewing night the epoxy joint failed at the epoxy/steel interface. I believe the steel must have contracted more than the plywood in the chilly temperature and sheared right off the relatively immobile epoxy/plywood interface.
Hank, I'm not sure, its been too long since I read the products manual, but it probably say something like one year. However, I've never found that to be a problem. Back when I was ordering 5 gallon pail quantities I might go a couple of years before using it all then dropping down to 2 gallon, which is what I order now.
My late brother-in-law was a chemist with AO smith fiberglass pipe. I could turn to him for advice some, and I think he indicated that since there wasn't any solvents to evaporate the resin was very stable. If you look at how many activators they offer for different attributes some have more reactive components that I would expect could shorten that. However the one I use can be shipped without being hazmat. I would suggest that you call them. I've talked to their tec-support, and they were very good at answering and offering suggestion.
Lots of woodworkers that have never used anything other than the small quantities they can pick up at the hardware store complain about the price being too high. But if you buy 2 gallons, its not much higher than TB at that volume.
Amines are one class of hardeners. They react with air to turn dark, eventually black. I have some that is 10+ years old and it has significantly degraded and has become very slow to set and likely does not set to make a strong material anymore. A insignificant tiny amount of degradation will turn the hardener dark. I am not sure one can tell if it has degraded to where it should not be used by visual inspection.
Some of the other classes of hardeners can also degrade over time. Best to check product literature for expected shelf life.