From what I can glean so far, the chief advantage of mortise chisels seems to be their thickness; they are nice and strong for lever action. But I won't be chiseling out the whole mortise, only the corners. I'm wondering if mortise chisels have any advantage over standard bevel edge chisels for that task.
If so, would you get a chisel the full width of the mortise? Or perhaps one size narrower?
One more thing: I find myself wondering if it's not easier to get a straight, square edge if I start out with an oval mortise that is slightly less long than the finished mortise will be. Otherwise I would need to somehow chisel an edge that's exactly tangent to the semicircular edge of the routed mortise. Any thoughts?