I've already fielded some advice about alternatives to square (through)
mortises. But for now - at least until I find out what a pain they are -
I intend to try to make some.

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?