With or without a honing guide, a skew bevel or edge easily occurs. You have probably noticed that as the edge is hammered down the bevel pushes the iron along the mortice breaking the “chop” free. A skew bevel will inevitably twist the chisel during that push, no matter what.
Being at best an acceptably mediocre woodworker, I find mortising easier with a guide and a flat side mortise chisel that required no preparation before use. Won’t work for mortices >½” though.
Sorry Neil, didn’t see your 2nd message before hitting submint