The existing tire wear does look normal and that's a good sign. However, if everything is OK...why have 2 of the original Enasaves been replaced? That doesn't absolutely mean there's an issue, but that's enough to make you scratch your head and wonder why.
A 4-wheel alignment check is almost a requirement on these cars to make sure you don't have a
rare case of a rear axle out of spec. Personally, I would pony up the $89 and go get a
4-wheel alignment done now.
If you haven't seen this thread yet that explains the potential problem, check it out here:
Some Mirage rear axles are out of spec