In general, there really is a problem; I clicked through the new window and the regular one (i.e., without the mod) in turn, and I suggest the following.

1. Move the left "Ok" button closer to the right "Ok" button (almost flush with a small gap so as not to miss the click; do not change the position of the right button, meaning both should be in the lower right corner).
It will be easier to reach, and two buttons side-by-side will serve as a reminder that this is a new window, and that the right button keeps the focus on the right hero rather than the left, as in the regular window.
2. Bind the "Enter" key to the left "Ok" button so that it keeps the focus on the hero initiating the trade, just like in the regular window.
3. Accordingly, bind the "Esc" key to the right button. Although it would probably be a good idea to use the "Del" or "Insert" keys here, for example, if that is technically possible.

I think this option will allow for the effective use of the new window's advantages and make the adjustment as easy as possible, especially for those who alternately use HD-SoD and other versions.