If your main worry about Susannah Aurora is that it is too long, that doesn't seem to be a big deal. Maybe it's just me (my daughter's fn and mn are each 4 syllables), but that name seems totally reasonable. Both are familiar and fairly easy to spell (you could lose the h off the end of Susanna if that helps at all). Stick in a cute nn like Zuzu and you're good to go.
As far as family members being upset about getting second billing in the mn spot, that also seems like it's unlikely. And if they feel that way, it's their problem. You're honoring important people with names that have meaning and sound good to you, and anyone who quibbles about the final result just deserves a stare of incredulity.
FWIW, I loooove Susannah/Susanna.