And that is the concern - that despite an alignment that is the 'best they could,' your rear axle is still out of alignment and will continue to eat up tires. I'm going to guess that is why the Mirage was sold with low miles in the first place.
Mitsubishi Motors North America has already acknowledged that out of spec axles are to be replaced under 5 year/60,000 mile warranty coverage. For details, please be sure to read this:
Rear Wheel Misaligned? It's not necessary to read the entire thread to get familiar with the problem, but it's clear many dealerships have avoided fixing this problem. By talking to the right people early on in the car's life, Mitsubishi can take care of it and keep these cars on the road.
14,000 miles is an unacceptably low odometer reading to be replacing tires. If that is the case, you'll be selling your Mirage soon, just like it's previous owner.
I wish I could help, but the best I can do is give you some information and advice on how to proceed. I like my little Mirage and want it to last as long as it can. I hope yours does too.