Veeah, I think your solution is fine. It is easy to read, once you get the hang of it. Thank you for noticing that the termination indicator is not needed after a shape symbol (unless it is modified). Regarding your last question about the internal termination, I think it reads well. It might be helpful to list that long construction in the list of symbols:
$su_$$33o] Square u-shaped symbol with right-pointing arrow directly over it (Nemeth Code symbol)
With so many transcriber-devised symbols, as well as those uncommon half-bracket symbols, I might suggest pulling out the symbols used in the calculator windows as a separate topic heading on your Special Symbols page. The cell-5 heading could be something like "Nemeth Symbols used in Calculator Windows". Then you can omit "(Nemeth Code symbol)" from each description.
Difficult examples are why we're here. They are fun to analyze and discuss. Thank you for the puzzle.