Nice analysis! I think I may have something additional to contribute. I recall learning, many years ago, that a "closed section" is extremely good at resisting torque. The column of your mill is open at the back, thus not a closed section. Just for fun, you should try modeling the addition of a more-or-less continuous plate to attached to the back of the column and closing it. Maybe something like your rear plate, but going all the way to the top of the column. Shouldn't have to be ½" thick. ¼" would probably work wonders.
Disclaimer: I don't have an 0619 myself, and don't know what interferences or complications might crop up if a plate is added to the entire back of the column. If nothing else, you'd have to modify the arm that holds your DRO read head. All I can go by is what I see in your Picture 2.
Since you've added the internal bars, it should be easy to add a full cover (or even a plate to cover above the existing rear plate) by screwing it to the internal bars. Without them, adding a full cover would require drilling & tapping into the back edges of the casting. But if you want to model the idea of just adding a full cover without the internal bars, that might be of interest to others who have the same mill.
Hope this helps!