A quick update:
Still working the GPC/IDP branch, cleaning up some I-loads and moving some displays forward, but mostly just reading and thinking about how the displays where handled in the GPC, and in particular about "control segments". It seems I'll have to introduce this concept to provide a better handling of the displays and the processes that supported them, and as an extension, of the Major Modes and OPS, as it seems like the "user interface" was the base of the "applications software". Still need to read (and understand) more to be sure I'll not be replacing one set of problems with another, and end up in the same place or worst.
Another active work area, this one for the near future, is an improvement of the Mission Editor, to have it follow the MVVM concept a bit closer, hopefully allowing a separation of the actual mission data from the presentation of such data. Most of the work has been done, but the Payloads part is giving me a few headaches... it seems I sorted it out by improving the design: did away with all the controls in that tab, and now there's just have a big table with all the payloads, with each type having its parameters in its own window. A 2-for-the-price-of-1 solution, which will allow in the future the addition of mass information to the payloads (and the vehicle), and display it all, and the total, in the same place!