Thank you for all your very much appreciated advice. I ended up doing like you said. My idea for the RH comes from a recording where it is all quick figures as well. The Petrucci score for the harp part I was handed is on pages 559-560 (Paris appendix), but that very spot is exactly the same (following triplets are different).
Discovering that massive scene 4 came quite as a shock. I have the feeling it is less notorious (amongst harpists) than the act I, scene 2 excerpts ?