My car's battery is 1.5 to 2 years old but in that time I had a dead alternator so there was a period of time when the car was getting jumped constantly (10-15 times total) and probably a little bit more than a month when the car's battery was sitting completely dead. It's in the shop now and my mechanic (who I trust) is saying that the battery is just completely shot. I'm just wondering if all the aforementioned strain on the battery could legitimately severely decrease it's life as it apparently has???

Yes. The faulty alternator plus periods of inactivity drained the life of the battery. I've seen cases where a battery was changed while the alternator was bad, two weeks later they had to come back for a new battery.

Completly draining a battery is very bad for it. Especially doing it repeatly. What you needed was a deep cycle battery. Wouldve lasted longer under those conditions.