Stand heavy mirror on its corner

You'll probably have an easier time using a picture hanger than a screw, and it will leave your wall in better shape when you remove it. Picture hangers transfer the downward force into pressure into the wall because the nail is mounted at an angle. The screw, on the other hand, is being bent because you're acting as a lever, where the stud is the fulcrum, the drywall is just empty space, and the wire is the force applied at the end. Even if you get a stronger screw, you're still placing a significant twisting force into the stud.

You can't use your typical picture hanger for a task like this. The small hangers that come in most kits are rated to only 20-30 lbs. However, you can find hangers like this one that is rated to 100 lbs (and I'd still use two of them, since it will make adjusting for level much easier and reduce the risk of failure).

The other option is called a french cleat which are two interlocking rails, one that attaches to the wall, and the other to what you're hanging. As long as you install them both straight and level, you don't need to adjust the mirror for level, and it distributes the weight across a larger area, reducing the risk of a failure.

50lbs is definitely something drywall or plaster can handle, but absolutely go with using the studs. Just regular wood screws or decks screws are fine, 2" to 2 1/2" long maximum to avoid any unknown electrical or plumbing. You can also double the screws on top of each other for more security.

Using the hinges or rings as they are will keep the mirror more vertical & tighter to the wall, but you have to very carefully level the screws based on the hinges or rings being under some amount load or weight. They may be out of level with each other or one may also be bent higher than the other.

Using the hinges or rings as anchors for a hanging wire, which is their intended & normal use, will bring the top of the mirror out from the wall & pitch the viewing angle down a little. But you only need 1-pair of screws side by side in one stud & leveling is done by centering the wire on the screws.

The short wall studs could indicate concrete, brick or cinder block construction halfway up the wall. The whole garage wall should read this way & was typically done to withstand slight car impacts. If it's just a 2 to 4' area then it could be the remnants of a fireplace or chimney.