CStan
Your problem is that silly pig-sticker mortise chisel. I mean it. I can't stand those things - they are too wide front to back. I had one for a grand total of a week. Had the exact problem you're having. Picked up my Sorbys and problem solved.

Use a proper sash mortising chisel (you are basically mortising in sash-sized workpieces anyway) and all your problems will be solved. You need the kind that are one or so degrees off kilter - a parallelogram shape when viewed head on.

The whole thing looks too overwrought. I use two handscrews - one around the piece being mortised, another to clamp that handscrew to the bench. Your guide piece is too tall IMO but that's more a matter of taste. The one I use is a little wider and less than half as long as the one in your photo.