Epi pens usually have expiry printed on them. Their lifespan can be reduced through exposure to extreme temps, particularly heat. That being said, there is little risk of harm from using an outdated epi, just risk of reduced effectiveness. With benadryl, liquid caps work a bit faster, but degrade faster. Hard tabs can remain close to full potency well after expiry.
One trick for avoiding wasps (you likely know)...second rider gets stung. Another reason to be first in the singletrack. Thankfully, this year doesn't seem too bad for wasps, and this heavy rain will likely put a damper on their vigor. Have a great race!