This was the option I would suggest "if" you can open up the wall and gain access to the studs. I would not be too comfortable trusting the mantle to a lag in the stud face to not sag or pull out over time. The bigger the lag, the less stud there will be there to hold it. The way we have done these is to fabricate a steel plate or yoke that lags/bolts into the sides of the stud with a 1" steel pipe projecting out to be embedded into the mantle. The plate/yoke allows you to bear more of the weight on more of the stud. Thats a lot of leverage on that connection and as stupid as it may sound the scenario you have to think of is one day someone is trying to hang a wreath above the mantle and they step, or stand, on the mantle and it either tears off the wall or bends at best.
If you dont do any metal fabrication any shop could fab you either a flange or a yoke that would allow you to attach to the sides of the stud rather than lagging into the face. That would all be dependent on your ability to open the wall up at the mantle connection point.
There are other options, metal bracket as opposed to a corbel where you could simply drill through a stone and using a small standoff (piece of tubing) as a shim, plumb the brackets, screwing them to the studs. Brackets on top of the mantle, and so on. But many of them will likely look like what they are, a patch job to recover from poor planning.
These are often the situations we get put in, "Hey I got all this done, now can you make it look right". Its never fun.