I have to say it sounds like you did a pretty thorough assessment of things looking back at it. If I had to offer any advice, it would be exactly what others here have given me; assuming an extremely strong defensive position and calling the police is rarely a bad idea. Walking into the situation yourself (ie, without a vest, without an armed partner) is a lot more dangerous.
Assuming you could open the garage door from inside the house, from a defensive position, and observe the door was open and alarm still going off, I would say it would be smart to hunker down and call the police. My personal preference would be to get out there with my gun, clear the garage and shut off the alarm, but in the name of staying alive in the worst case scenarios, that might not be the best idea.