I think your solution of using the Nachschlag to show that "flip" ornament is fine, especially since it seems to be defined in the music! Be sure to identify it as such in your special symbols list.
The plus and circle above the notes are shown in Table 30 - the plus is dots 126, 12 and would precede the note, while the circle is dots 13 and follows the note. I know it's a bit odd to have one of them before and the other after - but that's how the Code lays it out!
Hope that helps!