Looks like I really didn't read your post, you did mention the cross brace and I didn't see it.
Just wondering, how do you dismount these and where did you get the cases?
I have soft side bags on mine and they load from the top, I'm not sure how you load the side opening cases.
Overall looks great.
[EDIT] I see where they are bolted on, so not exactly made to be quickly removed. Given the size and weight, I think you've got it all covered.