Can seasonal allergies shorten life expectancy?

Severity dependent. Allergies if sever and left untreated could lead to recurrent bacterial infections as well as lung conditions and or infections that would mimic the effects of being a chronic smoker. In this way, yes.
Allergies. Not likely. I am sure some days it must feel like they will. But, i would say no.
No. Seasonal allergies can significantly affect quality of life, but not quantity of life. Although allergies cannot be cured, there is effective treatment--see an allergist to find out how.